Factor out styling functionality

[uliska]

The frames and highlighters modules have got a nice mechanism for handling properties, using

• the with-options macro
• unified definition of styling properties with defaults and type checking
• definition and application of stylesheets
• proper cascading of Default => Option => Stylesheet =>local definition of properties
• hierarchical stylesheets through a parent property

This should be reconsidered, generalized, and moved to the stylesheets package.