Fix a possible crash when working with split-views and when the working-set is...

Fix a possible crash when working with split-views and when the working-set is active in multiple areas.

It doesn't seem to be safe to clear one whole area while working on another one, so bring back the old updating code which only selectively closes views.
Clear the whole area on a higher level while switching working-sets, to make sure we sublime doesn't break while updating the views.
CCMAIL: kdevelop-devel@barney.cs.uni-potsdam.de
BUG: 277961