...and not all projects parts of a project if it is closed.

Re-produce with:
  open a project A with subdirs (e.g. qtcreator.pro)
  open a subdir project from project A (e.g. cppeditor.pro)
  close project A
  --> The project part representing the subdir is announced as removed
      although it is still open.

The clang code model was the only affected user - affected translation
units were not parsed/updated anymore
("ERROR: ProjectPartDoesNotExistMessage").

Change-Id: Ia79341ce201e3b4aefff9f597920dbc6f7d67634
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>