set the modified flag when saving a file that doesn't support saving all of the things (e.g. annotations) but supports file swapping

Example, png files.

If you add an annotation and save and choose to lose changes, the annotation will still be around "in okular" but not on the file, so we need to mark the file as modified so that when the user tries to close he will get again the warning of "you're trying to save and will lose data" because otherwise the user may think the annotation actually was saved since he can see it