Make GtkSourceView scheme-style configurable

This commit adds a "color scheme" entry to preferences which controls
the GtkSourceView scheme-style property to allow the user toggle between
different color schemes installed in the system. This is particularly
relevant for syntax highlighting since the default scheme (classic) is
not great in all circumstances such as dark theme usage. In general this
is heavily user preference driven so this commit allows users to select a
scheme they like.
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.
settings, meldbuffer: Tidy up some instance use
preferences.ui: Remove unnecessary padding around combo label