Metatype: Remove the need for runtime-registration of 3rd party containers.

Replace that need with a macro so that registration of the
container helper conversions is done at the time of registration
of the container (usually when it is put into a QVariant).

Change-Id: I823fb3fdbce306ebc9f146675ac43724cec678d5
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>