Fullscreen Editor (new fullscreen mode for clean UI)

Organize Maximize/Fullscreen mess and add a new fullscreen mode with no UI

 * Maximize Editor: (old Ctrl+Up)
 * Full Screen Window: (old Alt + F11)
 * Full Screen Editor: new operator (Alt + F10)

It should fullscreen the editor and make it clean (no UI, no buttons).
After exiting it, it should return to the previous window state
(windowed/fullscreen).

This was originally intended for the multiview branch, but this
functionality also benefits non-stereo workflows, thus it can be
reviewed and committed independently.

Development notes:
* This includes cleanups in the code to sanitize the naming of
  fullscreen/maximize across the window/editor code.

 Differential  Revision: https://developer.blender.org/D678