Editor and Coder: support incremental parsers on the level of stylers and text attributes

[JurajKubelka]

We already have a core support for incremental parsers (that can be activated using GtTextualCoder useIncrementalParsing: true). To take advantage of it, text stylers need to be updated to reuse previously used text attributes while styling a new version of a text (code). It also involves changes in some text attributes.