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>

src/action.c

@@ -72,6 +72,11 @@ action(struct server *server, const char *action, const char *command)
 		if (view) {
 			view_minimize(view, true);
 		}
+	} else if (!strcasecmp(action, "Move")) {
+		struct view *view = desktop_view_at_cursor(server);
+		if (view) {
+			interactive_begin(view, LAB_INPUT_STATE_MOVE, 0);
+		}
 	} else {
 		wlr_log(WLR_ERROR, "action (%s) not supported", action);
 	}