Basic support for Move action

- Add Move as a default mouse binding for a Titlebar Press action
- Remove the hard-coded handling in cursor_button()

Example config snippet:

    <mouse>
      <context name="Titlebar">
        <mousebind button="Left" action="Press">
          <action name="Focus"/>
          <action name="Raise"/>
          <action name="Move"/>
        </mousebind>
      </context>
    </mouse>

include/labwc.h

@@ -430,6 +430,8 @@ struct view *desktop_surface_and_view_at(struct server *server, double lx,
 	double ly, struct wlr_surface **surface, double *sx, double *sy,
 	int *view_area);
 
+struct view *desktop_view_at_cursor(struct server *server);
+
 void cursor_init(struct seat *seat);
 
 void keyboard_init(struct seat *seat);