g_variant_lookup works like g_variant_get, so using a format string of "v" only
works if the type is actually a GVariant. Since none of the hints have GVariant
values, that means every g_variant_get with a "v" format will fail.

Fix all of the g_variant_lookup calls so that they either unpack a value
directly, or use a "@" prefix when it's more convenient to fetch the value as
a GVariant pointer.

In addition, in cases where we do fetch a GVariant, make sure that we
unreference it afterward.

and drop additional checks for min/max GLib versions, it's not needed

Now that all of the elements are in place to support (dynamically) the
action icons, we can use the 'key' in place of the 'label' when the client
sends an hint requesting it.

Some other minor issues were corrected in this change, such as ensuring that
we always show *all* widgets inside of the notification window so that when
a live notification window is updated, the action box state is not then
hidden from view.

Signed-off-by: Ikey Doherty <ikey@solus-project.com>