media source: introduce media source/tree API

Add an API to manage "services discovery" easily from UI clients.

A "media source provider" allows to retrieve media sources (each
associated to a services discovery module).

A media source holds a media tree, containing both the media detected by
the services discovery module and the media detected by preparsing.
Clients may listen to the tree to be notified of changes.

A client may retrieve a media source at any time and listen to its
media tree, even if it is already populated, without race condition.

Signed-off-by: Thomas Guillem <thomas@gllm.fr>