shell-global: Fix a few memory leaks

https://bugzilla.gnome.org/show_bug.cgi?id=710104
shell-global: Fix an invalid memory botch-up

The value we return is a floating ref, which gjs shouldn't
try to free.

https://bugzilla.gnome.org/show_bug.cgi?id=710104
shell-global: Use G_VARIANT_TYPE macro for checking