Foundations for CSS OM

[devongovett]

Part of #5, #10.

This PR implements the foundations for supporting the CSS OM API. In particular, it provides metadata and functions for properties to convert between shorthand and longhand representations, as well as deal with logical property groups. It also implements get, set and remove methods for DeclarationBlock, which correspond with the getPropertyValue/getPropertyPriority, setProperty, and removeProperty methods of CSSStyleDeclaration. This allows retrieving any shorthand or longhand property no matter what declarations were actually written, as well as setting such properties.

The Shorthand trait is added to expose metadata about shorthand properties such as the list of longhands it includes, as well as retrieve and set longhands, or construct the shorthand from a list of longhand declarations. Most shorthands implement this via the define_shorthand or define_list_shorthand macros, which wrap around a struct definition.

cc. @lucacasonato this might help you.

To do

• More tests
• Ensure all shorthands are implemented
  • Grid
  • Mask border
• Improve docs