Added FindOrCreateWindowSettings() internal helper.

Docking: Reworked DockContextPruneUnusedSettingsNodes() to prune entire unused trees.

ImPool: Added Reserve() helper function.

Merge branch 'viewport' into docking

Merge branch 'master' into viewport ...

# Conflicts:
#	examples/imgui_impl_win32.cpp
#	examples/imgui_impl_win32.h
#	imgui.cpp

Renamed SetScrollHere() to SetScrollHereY(). Kept redirection function (will obsolete).

BeginChild(): Fixed BeginChild(const char*, ...) variation erroneously not applying the ID stack to the provided string to uniquely identify the child window. This was undoing an intentional change introduced in 1.50 and broken in 1.60. (#1698, #894, #713) + reworked the Begin/BeginChild comments in imgui.h.

Docking: Fixed tab order restoring (follow up to 67be485e, broken in 455dc6e2)

Docking: Added DockBuilderForkNode() primitive for usage for user code to fork/copy entire layouts. This will probably need to be added to as I find ways to migrate patterns from user code to the library. Added IMGUI_DEBUG_DOCKING_INI helper to facilitate debugging. (+1 squashed commits)

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)

ImVector: Fixed a oddly unqualified return type in the assignment operator (I assume C++ handles it nicely as this never warned anywhere, but it is completely unintentional).

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.

Viewport: Misc tweaks.

Examples: Vulkan: Fixed some minor discrepency in the SDL+Vulkan example to match the Glfw+Vulkan example. (cc #2097)

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.

ImDrawList: Fixed AddConvexPolyFilled() undefined behavior when passing points_count smaller than 3, in particular, points_count==0 could lead to a memory stomp if the draw list was previously empty.

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

Examples: Referring to missing gamepad support in back-end that are missing it.