ProjectExplorer: Fix crash on renaming

Nested event loops rearing its ugly head. Avoid all the trouble by
opening the message box on the next iteration of the event loop.

Task-number: QTCREATORBUG-13428
Change-Id: I3c06d649464c34875da9601fb2e13227a0397836
Reviewed-by: Robert Loehning <robert.loehning@theqtcompany.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>