Appearance: use settings.create_action

[danirabbit]

Fixes #429

Rebase merge/review commits separately

• Bind color scheme radios to color scheme gsetting with create_action
• Bind dark schedule radios to dark schedule gsetting with create_action

Removes a bunch of complicated touching of accounts service and simplifies settings binding. When manually changing color scheme while schedule is enabled, schedule will be snoozed instead of changed to disabled. Settings Daemon now handles setting mutter settings, syncing accountsservice etc