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/config/rcxml.c

@@ -465,6 +465,7 @@ load_default_key_bindings(void)
 static struct {
 	const char *context, *button, *event, *action, *command;
 } mouse_combos[] = {
+	{ "TitleBar", "Left", "Press", "Move", NULL },
 	{ "TitleBar", "Left", "DoubleClick", "ToggleMaximize", NULL },
 	{ "Close", "Left", "Click", "Close", NULL },
 	{ "Iconify", "Left", "Click", "Iconify", NULL},