[UPNP/JSONRPC] Allow usage of path that are inside sources + add source

[UPNP/JSONRPC] Allow usage of path that are inside sources + add source
setting allowSharing

Continuation of 1220d8c4a9
Add a source flag allowsharing defaulting to true to allow or disallow
access to a source from UPNP/WebServer

The checks are now available as a function in FileUtils and common to UPNP and JSON
Also make the check correctly works in JSON as current implementation have flow allowing browsing any directory with Files.GetDirectory.