Allow for Relative Positioning in Menu Files

[drewmalin]

Currently, only absolute pixel positioning is allowed in menu files. For example, to place a button at a particular location, you would write the following:

<button>
   <x>10</x>
   <y>53</y>
</button>

This works, but once the user gains the ability to modify the graphics settings, absolute positioning will no longer be as useful as it is now. We need the ability to say something like "place the button in the middle of the screen" rather than "place the button at (10, 53)." Something like:

<!-- Absolute -->
<button>
   <x>10px</x>
   <y>53px</y>
</button>
<!-- Relative -->
<button>
   <x>47%</x>
   <y>28%</y>
</button>