highlighting: added API call to query GeanyLexerStyle by name

[lpaulsen93]

This PR adds a new API call to highlighting.c:

const GeanyLexerStyle *highlighting_get_named_style(const gchar *named_style);

This shall enable plugins to query styles dedicated to plugin specific highlighting. Plugins could have their own styles with dedicated keys/names and instruct users to simply add them to a colorscheme. Another option would be a configuration setting in a plugin but then the setting is separated from the colorscheme which may result in bad readability.

Also see the discussion at #2331.

[elextr]

The use-case makes sense now that its explained clearly.

[codebrainz]

To be honest I dislike the idea of exposing anything colour scheme related to plugins as the current system is rather a mess due to how it evolved, and I'd very much like to be able to overhaul it - something I started some time ago, and will eventually continue with - without regard for breaking plugins (in addition to user schemes).

That said, I don't want to block progress for something that will not happen any time soon, so whatever.