[gi] Support nested objects and empty sequences in GLib.Variant building

The GVariant constructor (in the overrides) previously did not support empty
arrays/dictionaries or nested structures. Rewrite the VariantCreator class to
be fully recursive and determine the element types of arrays/dictionaries.

This now also allows you to use actual tuples as input values for GVariant
tuple types. Taking values from the flat argument list is still supported for
convenience, though.

https://bugzilla.gnome.org/show_bug.cgi?id=639939