xwayland: Process move-only client configure requests more quickly

xwayland: Process move-only client configure requests more quickly

For a move-only client configure request, treat it similarly to
view_move() by updating the scene-graph immediately, rather than waiting
for handle_commit().  Move-and-resize requests are handled the same as
before.

This (mostly?) fixes the glitchiness that was noticeable when dragging
an undecorated XWayland window (e.g. Audacious in Winamp mode).

Also:

- Reduce some code duplication in handle_request_configure() by
  simply calling configure(), as suggested by @johanmalm in #428.
- Factor out common logic after a move and/or resize into view_moved().