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)