Basic support for Move action

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>