lua-factory: Add grl.unescape() and grl.decode helpers

grl.decode will decode '%' encoded strings (usually URLs),
and grl.unescape will unescape HTML-escaped strings, usually
unicode or reserved characters.

https://bugzilla.gnome.org/show_bug.cgi?id=725381
lua-factory: Fix warnings when resolution fails

If the resolution fails, or the media passed from the callback
function is not in the right format, we need to pass back the
untouched original media, not NULL.
lua-factory: Factor out redundant code in lua_plugin_source_all_keys

The loop that gets the key names, checks if they exist, and adds
them to a list was repeated 3 times. Factor it out instead.
lua-factory: More warnings with unknown/unhandled metadata keys

Print out a warning when a key name is not registered, and upgrade
to a warning statement the debug output that warned about unhandled
GTypes.
lua-factory: Better warnings for unknown keys

So that script writers can see where the keys come from.
lua-factory: Fix "type" key in supported_keys

All those sources declared the "type" metadata key as supported,
but it's not actually a key. Remove it.

Also fix "mime" keys, it's actually called "mime-type".