Add .menu option for specifying the virtual screen size

[zturtleman]

To allow for true 16:9 Team Arena menu or HUD designs there would need to be a way to set the virtual screen size to be 16:9 (by specifying the width and height).

screenPlacement PLACE_LEFT PLACE_TOP allows a grid of 3×3 virtual floating areas (plus stretch options). However for a 16:9 design used in narrower aspect it would end up with overlapping elements as currently they are 4:3 floating areas. If the virtual screen size was 16:9 then in 4:3 it would fill width of the screen and spread out vertically.

(Note that screenPlacement currently only affects rendering only so mouse interaction doesn't work correctly.)