`PluginStore` should be subscribable, `withPluginEntities` should update props

`PluginStore` is currently used in the web interface to register entities which can be contributed by plugins. At the moment, it does not offer consumers to subscribe to its state and update them after a state change. Therefore, `withPluginEntities` is not able to update props after a state change of the `PluginStore` and the whole construct depends on the load order of different components.