glib: Add GVariant to fundamental types

GVariant is a fundamental type since 2.26, this commit adds it to the
fundamental type list and adds operators to construct and convert from
and to a GValue.

https://developer.gnome.org/gobject/stable/gobject-Type-Information.html#G-TYPE-VARIANT:CAPS

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>