Docking: Fixed dock node with single window accidental stealing of non-owned host viewport (leading to general havoc).

Docking: Added DockBuilderCopyDockspace() wip. Added DockBuilderCopyWindowSettings(), renamed DockBuilderForkNode() to DockBuilderCopyNode().

Added FindOrCreateWindowSettings() internal helper.

ImPool: Added Reserve() helper function.

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

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

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

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

Merge branch 'viewport' into docking ...

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

Docking: Internal renaming and comments. + Fixed DockNodeMoveChildNodes() to transfer the LastExplicitSize value (was inconsequential afaik).

Docking: Added ImGuiDockSpaceFlags_KeepAliveOnly, important for multiple level of tabs. (also renamed ImGuiDockFlags to ImGuiDockSpaceFlags.)

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

Nav, Focus: Fixed ImGuiWindowFlags_NoBringToFrontOnFocus windows not being restoring focus properly after the main menu bar or last focused window is deactivated.

Nav, Focus: Fixed ImGuiWindowFlags_NoBringToFrontOnFocus windows not being restoring focus properly after the main menu bar or last focused window is deactivated.

Docking: Updating LastFrameActive earlier in Begin() because BeginDocked() will need to use it. Extracted some code into a DockNodeIsDropAllowedOne() function. Comments.

Docking: Added SetNextWindowUserType() + user type filtering in dockspace and window merging. Added DockSpace() flag ImGuiDockFlags_NoSplit. Renaming. Reworked the DockNodeUpdateFindOnlyNodeWithWindows code so DockNodeUpdate can access the first window.

Merge branch 'docking2' into viewport_docking ...

# Conflicts:
#	examples/example_glfw_opengl2/main.cpp
#	examples/example_glfw_opengl3/main.cpp
#	examples/example_glfw_vulkan/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
#	imgui.cpp
#	imgui.h
#	imgui_demo.cpp
#	imgui_internal.h

Docking: Added Docking system. Enable with io.ConfigFlags |= ImGuiConfigFlags_DockingEnable. (Part 1) (#351)

Docking: Added ImVec2[] non const operator. Added ImStrSkipBlank. Reseting some values earlier in Begin. Added IMGUI_DEBUG_LOG() helper. Added docking source code section.