Easier support for new integrations

[vzahradnik]

Is your feature request related to a problem? Please describe.
At the moment, TcMenu allows extensibility through the plugins. However, the way how it works is not very straight-forward. We should think about how to improve this and to allow independent developers make their own extensions. Similar to the Home Assistant where the core code base is maintained by the core team and the integrations, albeit still part of the same repository, are maintained by external contributors. Each integration has a manifest with the contact to the maintainer who is notified about each pull request.

[davetcc]

All the code to do this is already in the designer, it can load plugins from anywhere on disk, we just need to make a new repository for "packaged plugins" that are not part of core, but easily installed afterwards. Now, any number of extra directories can contain plugins, we just need to make the conventions for it easier, and maybe create a third-party plugins repo.