PrefersColorSchemeSettings: add gsetting

[danirabbit]

Can be rebase merged

Fixes #14
Fixes #84

Needed to eventually fix #162

• Adds a new gsetting to set dark mode via settings daemon. This will cut down on duplicate code in Quick Settings and System Settings to set dark mode. Make sure we always handle setting things needed for Mutter, syncing to accountsservice, etc.
• Adds a new bool for snoozed state and set it automatically. This is so you can manually toggle dark mode on and off without canceling your schedule or having the schedule change the setting right back. Snooze will be disabled next time the schedule and color scheme are in sync or the schedule is disabled.

[zeebok]

Let me know if you want me to try and break things. I imagine you tested this pretty well though since it is a pretty important feature