Docking: Internals: Removed redundancy in code path leading to the BeginAsDockableDragDropSource(), clarified UpdateMouseMovingWindowEndFrame()

Note that the ConfigWindowsMoveFromTitleBarOnly path for UpdateMouseMovingWindowEndFrame() would previously test the window->RootWindow title bar instead of window->RootWindowDockStop. This didn't have any side effect afaik because we wouldn't enter that function anyway as clicking on any tab bar would trigger the move before UpdateMouseMovingWindowEndFrame() does it. However for consistency made the UpdateMouseMovingWindowEndFrame()code more correct.
+ minor renaming
3 files changed