Reload cached images that have been modified externally
This PR extends ContextManager's slotDirModelDataChanged to reload any cached documents that have not been modified by the user.
Performance impact of checking whether a document has been cached should be relatively low (two hashtable accesses); however, reloading documents on each change might be a bit overzealos since SortedDirModel's dataChanged can include items that have been re-sorted, and not just updated.
BTW, this MR was made during a livestream, starting at 2:42:27.
Edited by Bojidar Marinov