Ensure that first the editors unregister with the removed project part,
then unregister the project part.

This makes testing and following the message logs easier.

It also fixes the following race condition:
 * Unload project of opened file
 * Immediately close the editor before a new project part is calculated
 ==> The editor is closed, but the translation unit is not properly
     unregistered.

Change-Id: I618930d221323435ba60ac6c051380bccc9fdaf1
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>