view: validate move/resize request from clients

Move/resize requests from xwayland views and xdg toplevels should be
ignored when the view is not pressed.

This is relevant for touchpad taps with <tapAndDrag> disabled.

When the user taps the client surface (e.g. chromium and mpv) with the
setting above, libinput sends button press & release signals so quickly
that the compositor receives move/resize request from the client AFTER
the button release signal is processed, so `interactive_finish()` is
never called.

include/labwc.h

@@ -135,6 +135,10 @@ struct seat {
 	 * This allows to keep dragging a scrollbar or selecting text even
 	 * when moving outside of the window.
 	 *
+	 * It is also used to:
+	 * - determine the target view for action in "Drag" mousebind
+	 * - validate view move/resize requests from CSD clients
+	 *
 	 * Both (view && !surface) and (surface && !view) are possible.
 	 */
 	struct {