Fix bug #6606: Crash when reverting a document with a not available document class.

Fix bug #6606: Crash when reverting a document with a not available document class.

If the document class is not available, we now issue a warning for the user. This triggers a focusInEvent of the WorkArea and the cursor is issued to start blinking. However, when reverting a document the cursor is probably invalid and there has been no chance yet to fix it as we are still reading the file.

The solution is to not show the cursor when the view is still busy.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@34590 a592a061-630c-0410-9148-cb99ea01b6c8