editor with line-based highlighting

[mathisdt]

Make the editor aware of the syntax scheme (more than 50% whitespace means "chords", [these brackets] mean "translation") and highlight each part accordingly.

=== OR ===

Change the old highlighting and syntax scheme to an easier one, e.g.
... a normal text line
... {C} a chords line
... {T} a translation line

=== OR ===

Change the text stored in the XML to a structured form and create an editor which can change a line's type and does highlight it accordingly.

It should be possible to migrate the old syntax automatically into the new syntax.