preferences: Update syntax scheme preference to use combo helper
Shuffle around UI as asked by Kai

- Rename preference to 'Syntax highlight color scheme'
- Move preference under the syntax highlight checkbox
Move property handling to settings.py

As requested by Kai, removed style-scheme property handling from
filediff.py and moved it to meld.settings. Now meld.buffer just listens
to the changed signal of meld.settings and just changes scheme as a
response. Also moved scheme lookup to meld.settings.