Blender Git Loki

Coding work while on the trip to london (based on Plumiferos wishlist);

- Icon previews for Images were created always for old files, which made
browsing (menus) incredible slow. Added a minor change in the flow, so
icons only get created when the user invokes loading images.
Andrea; you might check this, probably not al cases are covered yet?

- Compositor: the 'File Output' node now has a min/max frame for which it
writes files

- Compositor: fixed a very bad bug (even in 2.42a release) that made the
depsgraph for nodes not work... while editing, only the nodes that change
should be recalculated, but accidentally all of them were done each time.