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.

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.

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.

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.

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.)

Nav: Added a way for code to cancel Alt for menu toggle (ImGuiNavInput_KeyMenu_). Cancelling out on platform window. close request. (#1542, #787)

Merge branch 'master' into viewport ...

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

Nav: Added a way for code to cancel Alt for menu toggle (ImGuiNavInput_KeyMenu_). Cancelling out on platform window. close request. (#1542, #787)

Docking: Added Type enum in ImGuiDockRequest. Renamed fields. DockSpace() skips node update if already submitted (when transitioning from implicit -> explicit DockSpace).

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.