[media] Added get method to request arbitrary key

New "get" method added to GriloMedia.

Now, it is possible to request the value stored in
an arbitrary key through its name.

The key can be a standard one from Grilo core or
another one dynamically added to the registry by a
plugin or application.
These are all going to be meaningless from QML and even if passed to native code that can do something meaningful proper reference counting of GObject's could be an issue.  I'd suggest just not handling those types, with the exception of variant where it might make sense to recursively extract the actual value.