Rename closeBuffer to closeWorkArea, because we'll have to await whether we'll actual...

Rename closeBuffer to closeWorkArea, because we'll have to await whether we'll actually close the buffer or not.

Now we can also pass the WorkArea we would like to close. This fixes also the problem that hideWorkArea still wasn't correct. That's a consequence of the fact that we relied on currentMainWorkArea(). I wanted to get rid of that anyway.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@31063 a592a061-630c-0410-9148-cb99ea01b6c8