[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.
Merge pull request #2764 from Tolriq/upnp_checksource

[UPNP/JSONRPC] add source setting allowSharing to secure source access from UPNP/WebServer