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)