Allow to create custom quick settings

This is a feature I always wanted on Android - to be able to create custom quick settings that can execute scripts I want. But now with Phosh, who cares about Android anyways. 😸

This MR provides a functionality to load custom quick settings as plugins. The overall working is based @guidog's comment.

1. We add an extension point similar to lockscreen plugin.
2. Create a key for quick settings in plugins schema.
3. Load (and watch for changes) the custom quick settings in PhoshSettings.
4. There is a sample custom quick setting named - PhoshSimpleCustomQuickSetting to test the changes.

TODO

• Clean up plugins/meson.build (currently we just piggybank on top of lockscreen plugins)
• Document how to test this functionality

Closes #859 (closed)