file-procedure.[ch] is gone and its functions moved to
GimpPlugInManager where they belong (the manager keeps around the
lists of load, save and export procedures).
Utility functions from file-utils.[ch] that have nothing to do with
image files moved to core/gimp-utils.[ch].
An image can not be considered both exported and imported at the same
time, so stop consider it as imported after we consider it exported.
Also add an automatic regression test.
Remove files in /tmp after tests so that other users on the same
machine (like buildbots) also can run make distcheck without getting
"can't overwrite file" errors.
