#678 Highlight escape sequences in strings

It would be nice if escape sequences (e.g.: "\0178") were colored slightly differently than other parts of strings (orange, perhaps).

See: https://sourceforge.net/p/geany/bugs/977/

[Anonymous]

This would need to be added to the LexHaskell.cxx file in the Scintilla project, at which point we could do the minor tweaks needed to Geany to use it (pretty much implicit when we update our Scintilla version).