dragsource: Reshuffle api a bit

dragsource: Reshuffle api a bit

Remove arguments from the constructor.

For actions, we now default to COPY, which is the most common one
that we should enable by default (MOVE requires handling deletion
on the the source side, and ASK only makes sense if we have
multiple actions).

For the content provider, we add a new ::prepare signal where
it should be provided just-in-time.