Setting up the library to support UIs and providers

This PR introduces a two new features:

- Providers: A compatibility version of DocumentsProvider has been added. A file system provider is implemented in a separate module. The providers can be easily used as SAF DocumentsProviders by using an adapter.
- Picker UI: A picker UI that can display the data from compatibility providers has been added. It is also in it's own module.

The modularisation of both providers and picker ui allows to easily integrate 3rd party plugins.