Plugin watchers don't work if another plugin is added later on

[dopenguin]

Describe the bug

In reverseGeocoder and pins the configuration parameters coordinateSources are being used to add watchers to react to changes in another plugins store. However, if the watched plugin is added with addPlugin after the plugin that is watching, the watcher can not be established.

Expected behavior

It should be possible to add plugins in any order and have watchers connect / disconnect on the fly.

Steps to reproduce

1. npm run snowbox
2. Setting a pin does not trigger a reverse geocode

[oeninghe-dataport]

Solution idea:

• Use a watcher on the list of installed plugins
• If the plugin is installed that should be watched, add the corresponding watcher.
• When the plugin is uninstalled, remove the corresponding watcher again.

The solution is needed in multiple plugins. This should be implemented generic (maybe src/composables).