Docking: Comments + maiintain LastFrameFocused per node + using bitfiield for docking bools.

Docking: Added context menu option to hide the tab bar of single-window dock nodes, similar to Unreal. (#2109)

Merge branch 'viewport' into docking ...

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

Viewport: Added support for minimized window which caused problem when ImGuiBackendFlags_HasMouseHoveredViewport was not supported. (#1542) + todo

Demo: Testing return value of BeginTabBar() for consistency.

Viewport: Added ImGuiConfigFlags_ViewportsDecoration to re-enable platform decoration (#1542)

Viewport: Added ImGuiConfigFlags_ViewportsDecoration to re-enable platform decoration (#1542)

Docking: Kept ImGuiDockNodeFlags_PassthruDockspace and removed ImGuiDockNodeFlags_PassthruInEmptyNodes, ImGuiDockNodeFlags_RenderWindowBg. Doesn't include ImGuiDockNodeFlags_NoDockingInCentralNode. (#2109)

Docking: Renamed "DocRoot/DocumentRoot" to "CentralNode", more self explanatory. Moved Splitter update higher up in DockNodeUpdate() + minor misc tweak. (#2109)

Docking: Renamed io.ConfigDockingWithKeyMode to io.ConfigDockingWithShift.

Docking: Better tracking of current dock id for inactive and uncreated windows (in settings etc.). + Fixed assert when docking a single-visible leaf node of a hierarchy into another

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

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

Refactor: Moved README, ChangeLog, TODO files to docs/ folder + update Changelog. (#2036)