x11, input: move mime type drag&drop negotiation to common code

Drag&drop mechanisms typically support multiple types for the drop data.
Move most of the logic which types are accepted and preferred to
event.c, where the data is also interpreted.

(Maybe sorting the types by assigning scores is over-engineered, since
they're already sorted by preference, but it's actually not much more
code.)

Not very interesting/meaningful yet, but preparation for the next
commit.

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