Improve fullscreen mode (bug 4146).

Improve fullscreen mode (bug 4146).

- added toolbars/tabbar/scrollbar toggling
- added text width control
- added GUI preferences & menu & shortcut

Known problems:
- Outliner in Mac is seeking some Mac devie to fix (see hints here:
  http://www.mail-archive.com/lyx-devel@lists.lyx.org/msg135382.html )
- Toolbars visibility is shared between windows (views)
  (needs toolbars overhaul).
- Adding new tab items does not reflect fullscreen mode
  (i.e. margins and tabbar get visible)

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@23069 a592a061-630c-0410-9148-cb99ea01b6c8