Avoid critical warnings when saving images on exit

This fixes multiple cricitical warnings when saving image while closing a window:

1. The general save callback and the specific callback for saving images on exit both clear the window's save job. The second callback triggering will then cause a critical warning. Instead of manually clearing the job, use g_clear_object which avoids that warning.

2. EogApplication could trip over an unexpected NULL pointer eog_window_get_image() when checking if a newly opened image is already open in any of the active windows.

3. EogListStore didn't properly cancel its thumbnail jobs while disposing. They could then asynchronously trigger the finish callback after the liststore was already finalized.

Fixes #263 (closed).