Relative sizing/positioning

[TangentFoxy]

I already knew I wanted this, but another quote from kikito that might help me think of what to do:

I mean that instead of saying "this button is 30px width and 50 px high", I want a gui
where I can say "in this group of buttons, each one is as wide as the container, and
25% its height". The only place I should have to specify pixel dimensions would be the
root "window"; this way, the Gui would work in any resolution.

[TangentFoxy]

Ideas:

• 0 to 1 means relative and is a percentage. Have optional boolean second argument that says to treat 1 as an actual value instead of percentage. This is stored separately from width and height. Default is for it not to be defined (well, actually false) if you're not using it, so it won't muck with absolutely defined things. Note: I may re-implement some of the primitives to default to percentage values instead of width/height values.
• Implement something to handle resizing of the game window, automatically re-aligning and re-sizing elements. The same function should handle anything internal being resized.

[TangentFoxy]

See #15