GVariant based API instead of GValue based

[Mikkel Kamstrup Erlandsen]

The 0.4 series uses GValue as generic container for data both internally and in the API.

This should be switched over to GVariant instead. This has numerous benefits:

 * We can have a single row setter dee_model_set_row (model, iter, variant)
 * More advanced getters dee_row_unpack (model, iter, column, signature, va_list)
 * We guarantee that all values are serializable (a GValue can hold any GObject)
 * Better/cleaner integration with GDBus and GSettings
 * The data container itself is refcounted (GValue is not)