Media Query support

Similar to CSS, media query support in UISS allows you to have layouts
specific to various device configurations (e.g.: iPhone 4 vs 5). This
code is also extensible to allow for additional properties (e.g.:
device-version: 6, device-version: 7) in the future.

Since UISS files are evaluated once at runtime, media queries can only
be used to evaluate device-specific properties.