Make setDefinition and setTheme virtual
Make setDefinition and setTheme virtual

This enables sub-classes to trigger a full re-highlighting for example.

janusw/syntax-highlighting,janusw/syntax-highlighting,janusw/syntax-highlighting,janusw/syntax-highlighting,janusw/syntax-highlighting,janusw/syntax-highlighting,janusw/syntax-highlighting,janusw/syntax-highlighting,janusw/syntax-highlighting,janusw/syntax-highlighting,janusw/syntax-highlighting,janusw/syntax-highlighting