Viewports, Docking: Added per-viewport work area system for e.g. menu-bars. Fixed DocksapceOverViewport() and demo code (overlay etc) (#3035, #2889, #2474, #1542, #2109)
Clarified that BeginMenuMainBar() had an incorrect knowledge of its height (which was previously harmless).
Designed to easily allow for status bars although we don't have/use them yet, but custom code could use them.