Gio::DBus: Use VariantContainerBase for tuple Variants.

* gio/src/dbusconnection.[hg|ccg]:
* gio/src/dbusproxy.ccg.[hg|ccg]: call_sync(), call(), call_finish():
Use VariantContainerBase, instead of VariantBase, for GVariants that are
documented as being tuples. Because the type is known we can also use
it as a simple return type instead of an output parameter.
* tools/m4/convert_gio.m4: Added a necessary conversion.
* examples/dbus/peer.cc:
* examples/dbus/userbus.cc: Adapted. These already correctly assumed that
the type should be VariantContainerBase.