Make backgrounds of ChatLines and ChatItems styleable

The background color (set by UiStyle) is now available in the MessageModel
via the BackgroundRole. It is used by the items to paint themselves, if it is
set by a stylesheet.

Message labels (so far, we support "highlight" and "own message") are also respected
properly. They can be accessed via the MsgLabelRole and are now used for accessing the style
engine.