Add preliminary label support to the style engine

Labels are supposed to be used for things like highlights. We now support
up to 16 different labels for defining special format settings overriding
the stock ones. This is a more flexible approach than hardcoding, say, a background
color and will allow full styling even for highlights and possibly other special
messages. For example, nick coloring can be adjusted to the highlight color via
stylesheet (once stylesheet support arrives).

Also, rework the format merging code to be much easier and still more flexible.