Fix for Editor Fullscreen on Sequencer

The sequencer 'display' area is a region on its own, so we can't hide
the preview regions. The only problem is that the <-> resize icon shows
in the main region, so you only see it over the tracks region in the
sequencer, I'm not addressing this though.
Merge remote-tracking branch 'origin/master' into multiview

Note: recent commits to UV/Image Editor were not tackled yet, so I
suspect the view menu is not fully functional at the moment.

Conflicts:
	release/datafiles/locale
	source/blender/blenloader/intern/versioning_270.c
	source/blender/editors/space_image/image_buttons.c