backend-wayland: replace a nested wl_display_roundtrip call

The wl_display_roundtrip call was originally introduced to let the
display_add_seat function wait until a wl_seat.name event was received.

This change replaces the wl_display_roundtrip call with an
asynchronous, nonrecursive equivalent. Now a wl_display.sync callback
is used to delay the final steps of adding a seat until one protocol
roundtrip has occured/the name has been received.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>