GSimpleAction: don't allow changing state type

g_object_set() allowed us to bypass the usual checks that the state
doesn't change type and also leaked.

Fix that up by turning the state into a construct property (so that it
always gets set once during construction, even if only to NULL) and
then route the further sets through the C API so that they are subject
to the same checks.

https://bugzilla.gnome.org/show_bug.cgi?id=696424
exporter: give error on Describe of missing action

If someone calls org.gtk.Actions.Describe on a non-existent action then
return an exception instead of a trivial description (disabled, no
state, etc.).

https://bugzilla.gnome.org/show_bug.cgi?id=687185