Plugin PBW support

- Support loading plugins from PBWs. These can either be alongside an app/face or alone
- They are stored in the locker table, with a new type of app 'plugin' when there is no watch-side binary
- Add a plugins tab on the Apps page (only when plugins are enabled) to allow configuring/uninstalling them
- Plugin registry is populated from locker db table (plugin js scripts are lazily loaded when required)
- Remove bundled raw plugin support - all bundled plugins are now pbws (built at app-compile-time)
- Move some plugin manifest fields into the main appinfo json, and make the new config pages replace legacy config pages, when defined in json (still falls back to old config pages if not)
- Added a script to enable building plugin PBWs (until they get support in the Pebble tool)
- Refactor native kotlin plugins under NativePlugin interface (will be used in future to support config UI for these)
- Lots of refactoring/tinkering to make things feel right now that plugins are "fully" supported as apps