Add position arguments for menus

Add position arguments for menus

...utilizing x,y coordinates where values can be a number, a negative
number, a percentage or "center".

- (0,0) is top left corner
- (-0,-0) is bottom right corner
- % is percentage of width and/or height
- 'center' centers the menu vertically and/or horizontally

    <action name="ShowMenu">
      <menu>root-menu</menu>
      <position>
        <x>0</x>
        <y>0</y>
      </position>
    </action>

Note: both x and y values must be supplied for positioning to work.