Gio::Action: #include <glibmm/variant.h> in hg

Currently, it is included indirectly, via 3 other headers. This is
brittle. Without that, users of action.h would have to manually include
variant.h to use methods returning the forward-declared VariantBase,
which isn’t very helpful. Plus, the header uses types like Variant<>
templates and VariantContainerBase, which were never forward-declared.

Including it directly makes the dependency more obvious and less brittle
and spares us from having to write a bunch more forward-declarations.

https://bugzilla.gnome.org/show_bug.cgi?id=777953