Turn SettingsBridge config options into properties

Make SettingsBridge properties actual properties instead of using getter and setter functions. This enables proper NOTIFY signals, which makes all changes immeediate. For example, preferring Signal/Sailfish contact profile data has immediate effect when navigating back to main page.

I couldn't find a neat way to auto-implement the repeated functionality, so this is awfully repetitive at times. At least it ensures all desired values are taken care of properly.

Fixes #1 (closed)