Highlight scheme

Highlight and auto-complete scheme code.

Add new scheme colors in the colors preference page:
* Keyword (The most common guile procedures and keywords)
* Function (All the public functions  that are in the scm directory of lilypond and the ly:* functions)
* Variable (All the public variables  that are in the scm directory of lilypond)
* Constant (All the public constants  that are in the scm directory of lilypond)
* Symbol (scheme symbols that start by a quote)
* Delimiter (scheme parentheses)

Add all these words to the completer

Fixes https://github.com/wbsoft/frescobaldi/issues/145