replace all uses with const char * (non-interned).
Also remove a lot fo juggling from atom to GdkAtom to string and back.
The X Atom hash table is now mapping to (again, non-interned) strings.
(1) Try all passed in formats in order if one of them fails.
(2) Don't blindly accept all formats, make sure they are mime types
(3) Add a bunch of special non-mime types that plug converters to
get to mime types
