Docking: Fixed losing tab bar selection when extracting a whole docked node + reusing existing dock node when possible.

Merge branch 'viewport' into docking ...

# Conflicts:
#	examples/example_glfw_opengl2/main.cpp
#	examples/example_glfw_opengl3/main.cpp
#	examples/example_glfw_vulkan/main.cpp
#	examples/example_sdl_opengl2/main.cpp
#	examples/example_sdl_opengl3/main.cpp
#	examples/example_sdl_vulkan/main.cpp
#	examples/example_win32_directx10/main.cpp
#	examples/example_win32_directx11/main.cpp
#	examples/example_win32_directx12/main.cpp
#	examples/example_win32_directx9/main.cpp

Docking: Fixed undocking on whole dock node from leaving undesirable empty node in the docking tree. Fixed calls to MarkIniSettingsDirty().

Nav: Use Platform_SetWindowFocus when CTRL+Tabbing to another viewport.

TabBar: Recover if SelectedTabId doesn't exist anymore.

Viewport: GLFW: Build fix for pre GLFW 3.2 (#1542)

Viewport: Enabled viewports by default in more example apps.

Docking: Renamed ImGuiDockSpaceFlags to ImGuiDockNodeFlags. Clarified in comments/demos that DockSpace creates a Node. Renamed IsExplicitRoot to IsDockSpace. Assert against explicitly calling DockSpace twice in a frame.

Docking: Added undocking of whole dock node by dragging from the Collapse button. Super useful and works great!

Docking: Fix of title bar flicker during rebuild. Fixed dragging of DockNode always triggering BeginAsDockableDragDropSource() when io.ConfigDockingWithKeyMod is false.

Docking: Internals renaming LastExplicitSize to SizeRef (invalidate .ini data) + todo

Docking: Creating tab bar and adding window into tab bar immediately (not sure why I didn't do that before). Probably broke something subtle, will find out. *Edit* Initially broke restoring selected tab.

Docking: work to allow programmatic control of dock nodes, various refactor + assert fix. Probably broke something (but I haven't found what yet!)

Docking: Internals: Transitioning some code toward consistently using ImGuiContext* ctx parameter instead of ImGuiDockContext

Docking: Fixed central docking display with ImGuiDockSpaceFlags_NoSplit, affecting in particular with io.ConfigDockingWithKeyMod = false. + comments, asserts

Fixed bad merge (a82be53) where we lost the version number in Docking branch + moved Docking related API together for now.

Docking: Changed DockSpace() signature to take an ID, as ID will frequently be used by other API it makes more sense to let the user compute it.

Docking: Removed SplitRatio from ImGuiDockNode and ImGuiDockNodeSettings, which we don't need anymore.

Docking: Misc rework/rename toward being able to rebuild a branch selectively, so we can honor settings changes on a per Dockspace basis. + Comments

Merge branch 'viewport' into docking ...

# Conflicts:
#	docs/CHANGELOG.txt
#	imgui_demo.cpp
#	imgui_internal.h

Viewport: Increased threshold for setting ImGuiViewportFlags_NoFocusOnAppearing. (#1542), 2 doesn't seem enough with some docking setup.

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_dx9.cpp
#	examples/imgui_impl_opengl2.cpp
#	examples/imgui_impl_opengl3.cpp
#	examples/imgui_impl_vulkan.cpp

BeginMenu(): Fixed menu popup horizontal offset being off the item in the menu bar when WindowPadding=0.0f.

ColorPicker: Replaced the Separator with a Spacing call. (#2068)

Viewport: Fix an issue introduced on ~August 16, which would assert when viewport are disabled. Also made the DestroyPlatformWindow process more sturdy. (#1542)

Docking: Fixed floating->docking transition on a platform window mistakenly destroying the platform window because the upcoming tab isn't visible and triggers viewport GC. Fixed missing title bar data on recreate window.

Examples: Using "dear imgui" terminology in all examples headers/comments + fix minor typo.

Docking: Better handling of window losing its dock node or having its dock node not active.

Docking: Flicker fix when clicking on a Tab leading of a new window, in particular would be noticeable when using nested tab bars.

Fix MSVC's C4312 warning about casting user texture type to void* in ImageButton (#2080)