remove grouped property bindings from the language

Currently it's possible to use the syntax for an object declaration to group property bindings:

anchors {
  left: a
  right: b
}

This has multiple issues:

• potentially confusing: Users might think you are declaring an object of type 'anchors'. The capitalization is the only difference!
• two ways for one use: This style and the fully qualified style are two semantically equivalent ways of expressing the same thing.
• bad for tooling: Complicates rewriting from ui designer, limits completion in text editor.