media library: force a reload when sources have changed in files view
SourcesDirectory: avoid retrieving removable devices if getting media sources failed
media library: add UPnP servers to sources://<mediatype>
upnp: fire an update source message when a UPnP server becomes available/unavailable
make "Zeroconf Browser" localizable
Merge pull request #5492 from Montellese/upnp_server_as_sources

video library: list UPnP servers among video sources