If we're going to move or resize an xwayland surface, we used to just
pick the first seat in the list for doing the move/resize. Ideally we
can map from the XInput device doing the click to the corresponding
weston_seat, but that requires using xcb xinput, which isn't well supported.
Instead, lets use a simple heuristic that just picks the pointer that
most recently delivered a button event to the window in question.
Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=73807
Set up X windows that are transient for another window as transient
surfaces in shell.c. This keeps the transient windows on top of their
parent as windows are raised, lowered for fullscreened.
https://bugs.freedesktop.org/show_bug.cgi?id=69443
