Viewport: Avoid creating viewport on drag (fix cc882b0)

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/allegro5_example/imgui_impl_a5.cpp
#	examples/allegro5_example/imgui_impl_a5.h
#	examples/allegro5_example/imgui_impl_allegro5.cpp
#	examples/allegro5_example/imgui_impl_allegro5.h
#	examples/allegro5_example/main.cpp
#	examples/imgui_impl_allegro5.cpp
#	examples/imgui_impl_allegro5.h
#	imgui.cpp
#	imgui_internal.h

Internals: Removed unecessary Pos/PosFloat separation, only reason appears to be mostly pre-1.0. The only piece of code that I expected sub-pixel window position to matter actually already round its delta (wrongly so, will fix later/separately if we want).

Viewport: Tidying up AddUpdateViewport() calls. Moved flags argument. Removed extraneous mid-frame call to FindPlatformMonitorFromRect(). (#1542)

Viewport: Fixed glitches with newly appearing menus creating viewport while the underlying imgui window should be hidden. (#1542)

Viewport: Fixed issues with popups drifting in particular when reference mouse position become invalid (e.g. changing app focus while viewported-menu is open). Storing monitor index in viewport at beginning of the frame. (#1542)

Fix various typos.

Viewport: Fixed mouse hover flicker on mouse button release frame after moving a window. (#1542) ...

Platform: Clarifying some comments in back-ends.

Removed presumably obsolete MovingWindow tests which prevent move/merge logic in viewport branch from working in all situations (e.g. docking away when ActiveId is the ID of a tab)

Removed presumably obsolete MovingWindow tests which prevent move/merge logic in viewport branch from working in all situations (e.g. docking away when ActiveId is the ID of a tab)

Internals: Removed misleading ImRect::FixInverted + fix minor formatting in Readme. ...

Demo: Fixed Overlay: Added a context menu item to enable freely moving the window.

Viewport, Platform: Fixed IME positioning for multi-viewport. Moved API from ImGuiIO to ImGuiPlatformIO. Because it is extremely unlikely to people redefined this API manually the moving-forward-breakage is ok. (#1542) ...

SDL2 ime support under Win32 never worked properly because of SDL interferences.

Viewport: Clamp windows within monitors + fallback rescue window when it is out of sight (e.g. removed monitor, changed resolution) + Win32: declare primary monitor at the beginning of the list. (#1542)

Viewports: Todos, Tooltips/menus not automatically forced into their own viewport when the NoMerge flag is set (however the multiplication of viewports makes them more likely to protude and create their own). Win32: try to make primary monitor the first tin the list. (#1542)

Viewport: Added ImGuiConfigFlags_ViewportsNoMerge flag (to enforce a platform window for all floating windows) + minor tidying up and addition of non-functional wip code.

Window: Relaxed the internal stack size checker to allow Push/Begin/Pop/.../End patterns to be used with PushStyleColor, PushStyleVar, PushFont without causing a false positive assert. (#1767)

Viewport: Fixed initial popup positioning not using the monitor area properly. (#1542)

Viewport, Platform: Added work area in ImGuiPlatformMonitor. Renamed fields. (#1542)

InputText: On Mac OS X, support Cmd+Shift+Z for Redo. Cmd+Y is also supported as major apps seems to default to support both. (#1765)

InputText: tweaks, minor refactoring, renaming + comments (#1695)

Viewport: Switched to using unified platform-absolute mouse coordinates, which simplify lots of problems and simplify/reduce tricky conversions, makes ImVec2 less ambiguous. Fixed various viewport/windowing/popups/synchronization bugs. Settings on host-viewport are stored as relative (made settings decently compatible between viewport enable/disabled settings). Merged ImGuiViewport::Pos and ::PlatformPos. Tweaked thumbnails. Better, smaller code. (#1542)

Columns: Fixed a bug introduced in 1.51 where columns would affect the contents size of their container, often creating feedback loops when ImGuiWindowFlags_AlwaysAutoResize was used. (#1760)

Window: Fixed windows using the ImGuiWindowFlags_NoSavedSettings flag from not using the same default position as other windows. (#1760)

InputText: Tweak to make the if statement more readable + update Changelog (#1747)

on OSX, ignore text input when CMD key is down

Viewport: WIP for Tooltips, Popups, Menus to create their own viewport. Resizing a window allows it to leave the main viewport. (#1542)

Documentations for Github users. Metrics: Fixed display of NavRectRel, added display of ParentWindow.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_dx10.cpp
#	examples/imgui_impl_dx11.cpp
#	examples/imgui_impl_dx12.cpp
#	examples/imgui_impl_dx9.cpp
#	examples/opengl2_example/imgui_impl_glfw_gl2.cpp
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/sdl_opengl2_example/imgui_impl_sdl_gl2.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/vulkan_example/imgui_impl_glfw_vulkan.cpp
#	imgui.cpp
#	imgui_internal.h

Fixed DisplaySafeAreaPadding not affecting popups/tooltips/menus at it should have (partly broken in 2e5577f).

BeginMainMenuBar: Followup to 1e41bad9. Removed public window flag. DisplaySafeAreaPadding defaults to (3,3) instead of (4.4). Comments, tab to spaces, Changelog. (#1439)