Implement syntax highlighting in diff editor

All Qt Creator's main highlighters are used in the first place,
for other mimetypes generic highlighter is used as a fallback.

Task-number: QTCREATORBUG-9580

Change-Id: I863b9085520e5bdda142ce88f2074afeacee0531
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>