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.h

IO: Renamed io.WantMoveMouse to io.WantSetMousePos (was added in 1.52, _not_ used by core and only honored by some binding ahead of merging the Nav branch) + internal renaming (#787)

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/sdl_opengl2_example/main.cpp
#	examples/sdl_opengl3_example/main.cpp
#	examples/vulkan_example/main.cpp

Viewport: Changelog + Internals: Renamed viewport mouse fields for consistency (going to add more). (#1542)

Nav: Track nav input source more generally (gamepad vs keyboard) (#787) + update todos and demo tweaks

Viewport, Platform: Refactored platform interface. Removed need to use imgui_internal.h in backends. Split viewport into public facing ImGuiViewport and internal structure. Exposing enough data to provide custom tweaked renderers. Renamed handlers, fixed lots of inconsistencies. (#1542, #1042)

Viewport, Platform: Internal renaming to use more consistent (OsDesktopPos > PlatformPos). (#1542)

Viewport: Removed back-end renderer code that implied we need to clear with WindowBg color. Added NoRendererClear flag. (#1542, #1042)

Viewport, Platform, Examples: Changes to resizing flow + restored support for Platform events affecting the ImGui windows (so Decorated windows are functional). (#1542, #1042) .. ...

SDL: Added platform move/resize/close support.
GLFW: Added platform move/resize support. Moved Close to use callback for consistency.
Win32:
Vulkan: Fixed resize support.
Naming is WIP "PlatforrmRequestXXX" is too ambiguous. Basically we either have a ImGui->Platform flow or a Platform->ImGui flow. Working a bigger refactor now.

Metrics: Displaying all draw lists of a window (if it uses layer 0 and has already been appended to earlier in the frame).

Merge branch 'master' into viewport

Moved IM_NEW, IM_DELETE helpers to imgui.h for convenient use by back-ends and user code.

Merge branch 'master' into viewport (inc Vulkan resize fix merge #1042)

Internals: Columns renaming fields

Viewport, DPI: Some early work on per-viewport DPI support. At the moment the easiest way is to replace fonts during the ChangedViewport callback, but down the line we should aim at handling some of it at ImFont level. (#1542, #1676)

Viewport, DPI: Select viewport before locking style sizes and before handling double-click-on-title-bar to collapse. (#1542, #1676)

Merge branch 'master' into viewport

Viewport,Platform: Added GetWindowDpiScale() platform interface, changes of scale are reflected by positioning and resizing windows in their given viewport. (#1542)

Viewport: Coupling non-main viewport to one window. Viewport name derived from the window. Added ImGuiConfigFlags_PlatformNoTaskBar, off by default (aka re-established task-bars by default for now). (#1542)

Drag and Drop: Increased payload data type to 32 characters. (#143)

Viewport: Disable extra viewports from hosting other windows. (#1542)

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_glfw.h
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.h
#	imgui.cpp

Viewport: Tracking current viewport we are appending to + added callback in PlatformInterface for DPI purpose (WIP). (#1542)

Columns: Extent stop at the right-most clipped pixel. The right-most column might appear a little wider but it's usable space matches the others. (#125). +9 Internal: Store InnerClipRect.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/allegro5_example/main.cpp
#	examples/directx10_example/main.cpp
#	examples/directx11_example/main.cpp
#	examples/directx12_example/main.cpp
#	examples/directx9_example/main.cpp
#	examples/imgui_impl_dx10.cpp
#	examples/imgui_impl_dx11.cpp
#	examples/imgui_impl_dx12.cpp
#	examples/imgui_impl_dx9.cpp
#	examples/imgui_impl_glfw.h
#	examples/imgui_impl_vulkan.cpp
#	examples/opengl2_example/imgui_impl_glfw_gl2.cpp
#	examples/opengl2_example/main.cpp
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/opengl3_example/main.cpp
#	examples/sdl_opengl2_example/imgui_impl_sdl_gl2.cpp
#	examples/sdl_opengl2_example/main.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/sdl_opengl3_example/main.cpp
#	examples/vulkan_example/main.cpp
#	imgui.cpp
#	imgui.h

Viewport: Give a chance to platform and renderer to actually destroy their data. Otherwise the regular ImplXXX_Shutdown + following by DestroyContext() order fails to fullifl this. (#1542)

Renamed ImGuiStyleVar_Count_ to ImGuiStyleVar_COUNT for consistency with other enums. Added

Added ArrowButton(). Exposed ImGuiDir.

Internal: renamed RenderTriangle() to RenderArrow().

Internal: Moved IM_NEWLINE helper to imgui_internal.h