[syntax_highlight] Fix deprecation warnings and restructure code
This fixes some deprecation warnings related to Gtk.Color and restructures code and formatting according to pylint suggestions.
I added a ScrolledWindow to the PluginConfigDialog's test TextView so it won't expand into eternity.
@FlorianMuenchbach I started out fixing these deprecation warnings but after my linter lit up like a christmas tree I thougt I might as well restructure the code according to the pylint suggestions. These changes are quite huge, please test them thoroughly !