Docking: Minor renaming. DockContextAddNode() uses 0 as marker for automatic ID selection + TODO update and moved docking entries to docs/TODO.txt

Docking: Renamed SetNextWindowDock() to SetNextWindowDockId(). Added GetWindowDockId().

Docking: Reworked SetNextWindowUserType() as SetNextWindowDockFamily(), allowing multiple scene to be tagged with their scene id and not receive tabs from other scene + persistent storage for family id to allow for docking tree fork/remap.

Docking: Added DockBuilderGetNode() wrapper for consistency.

Docking: Added DockBuilderRemoveNode() + various tweaks. Fixed dragging/undocking dock node from CollapseButton.

Docking: Fix for undocking node where the dockspace node is attached to an immovable parent window. + Tweak .ini output.

Docking: Some DockBuilder functions are applied on settings data if windows are not present. Added DockBuilderCreateNode which needs a size else if we can't split properly. DockNodeTreeSplit() doesn't clamp SizeRef. (+1 squashed commits)

Docking, Viewport: Reworked viewport inheritance/transition so that extracting a dock node from a split node owning its viewport will have both viewports be in the expected OS z-order. (The lower window re-use the exiting viewport, the detached node uses a new viewport:

Docking, Viewport: Rework ownership transfer/stealing to fix issues where non-child windows could be stick marked as not owning their viewport.

Docking+Viewport: Fixed PlatformRequestClose (e.g. ALT-F4) being redrected to the first window of the docking node instead of the selected one.

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