Allow plugins to have prefrences

These will be used by phosh-mobile-settings.

Marking as draft as it depends on !1168 (merged)

TODO

• actually set gsetting for ticket-box
• fix styling
• improve tools/plugin-prefs

Mostly putting it out so plugins like !1165 (merged) can work on the UI.

Usage:

PLUGIN=ticket-box _build/plugin-prefs