wayland: write wl_data_offer data asynchronously

Currently writing wl_data_offer data into the fd is 1) synchronous, which
is noticeable when transferring large amounts of data, and 2) buggy, write()
error checking is done on the accumulator, breaking both the written data
accounting and error checking itself.

Fix both by making writes asynchonous through GOutputStream, the operation
is spun off and either finished, or cancelled if new data is stored in the
selection while the transfer is active.