Fix deadlock when closing gwenview importer
When gwenview_importer is closed after importing files, it gets into a deadlock. This bug was introduced in commit 4f7d35d1 by the refactoring of thumbnailprovider and thumbnailgenerator.
The deadlock is caused by the fact that closing the application will directly trigger a wait() in the thumbnailgenerator thread which will wait on proper closure of this thread before allowing the application to exit. However, this thumbnailgenerator thread is, at that moment, in a QWaitCondition state which is intended to wait on new thumbnails to be generated. As far as I can see, this is not a problem in the main gwenview application because the deadlock will eventually be lifted by the destruction of the thumbnailgenerator object by the chain of destructors going from application level down to thumbnailprovider which is the owner of this object. In the importer the situation is a bit different. After importing has finished, the thumbnailpage which is the owner of thumbnailprovider is not the current widget anymore. It still exists in the background in case the user presses the "Import more Documents" button. For some reason the destruction chain doesn't seem to properly trigger the cancel() function of the thumbnailgenerator object/thread.
The simple solution is to call the cancel() function just before the wait() function is called when the application emits aboutToQuit(). This is proper behavior, since the thumbnailgenerator should always be triggered to close down when the application is quitting.