(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
This allows us not just to pass any mime type to the read function, but
it also makes it possible to pass multiple mime types and the clipboard
can then try them in order until it finds a supported one.
This is so far not implemented though.
Turns out, way too many async operations are implemented by running the
sync operation in a thread. The easiest solution is to support that is
to use a GAsyncQueue for the buffers and deadlock if called from the
main thread.
(1) Turn X11 clipboard event handling into a regular filter function
(2) Maintain a timestamp in the clipboard, so we can pass it when
querying selections.
