core: Make sources network aware

If a source says that it requires the local network, or access to
the Internet to work properly, automatically hide it when the network
becomes unavailable, and show it when the network comes back on again.

This is implemented using GNetworkManager's connectivity property.
See https://bugzilla.gnome.org/show_bug.cgi?id=664562

Sources only need to set the "net:local" or "net:internet" tags to
be shown/hidden from applications depending on the network status.

https://bugzilla.gnome.org/show_bug.cgi?id=725148
core: Don't require a newer GIO for network awareness

As a separate commit, because it will make it easier to revert
once we're ready to bump the GLib/GIO dependencies.

https://bugzilla.gnome.org/show_bug.cgi?id=725148
Revert "core: Allow removing metadata keys in GrlData"

This reverts commit 32275f8daace57eaea1899ad534244091e05e0a9.

This patch broke adding empty strings as related keys, as the
lastfm-albumart plugin tests showed.

https://bugzilla.gnome.org/show_bug.cgi?id=741207