CppTools: Do not call GC() more than necessary

Currently GC() is invoked if
   - 5 CppEditors were closed or the last CppEditor was closed
   - a project is about to be removed
   - a session is about to be unloaded

Thus, for the following use cases, too much GC() calls (can) happen:
    - File > Close All
    - Close All Projects and Editors
    - Changing the session

Fixed by introducing a timer.

Change-Id: I9c984d9de735fc8c6ee77a518e9fb5b63dba5881
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>