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

docs/TODO.txt

@@ -127,7 +127,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - dock: B: when docking outer, perform size locking on neighbors nodes the same way we do it with splitters, so other nodes are not resized.
  - dock: B~ central node resizing behavior incorrect.
  - dock: B~ central node ID retrieval API?
- - dock: B- full rebuild (which is a debug option) loses viewport of floating dock nodes.
+ - dock: B: changing title font/style per-window is not supported as dock nodes are created in NewFrame.
  - dock: B- dock node inside its own viewports creates 1 temporary viewport per window on startup before ditching them (doesn't affect the user nor request platform windows to be created, but unnecessary)
  - dock: B- resize sibling locking behavior may be less desirable if we merged same-axis sibling in a same node level?
  - dock: B- single visible node part of a hidden split hierarchy (OnlyNodeWithWindows != NULL) should show a normal title bar (not a tab bar)
@@ -143,7 +143,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - dock: B- tab bar: make selected tab always shows its full title?
  - dock: B- tab bar: the order/focus restoring code could be part of TabBar and not DockNode? (#8)
  - dock: B- nav: design interactions so nav controls can dock/undock
- - dock: B- dockspace: flag to lock the dock tree and/or sizes
+ - dock: B- dockspace: flag to lock the dock tree and/or sizes (ImGuiDockFlags_Locked?)
  - dock: B- reintroduce collapsing a floating dock node. also collapsing a docked dock node!
  - dock: B- allow dragging a non-floating dock node by clicking on the title-bar-looking section (not just the collapse/menu button)
  - dock: B- option to remember undocked window size? (instead of keeping their docked size) (relate to #2104)