gobject: add 'explicit notify' GParamSpec flag

Add a flag to prevent the automatic emission of the "notify" signal
during g_object_set_property().

If this flag is set then the class must explicitly emit the notify
for themselves.  This is already standard practice on most classes, but
we cannot simply remove the existing behaviour because there are surely
many cases where it is needed.

https://bugzilla.gnome.org/show_bug.cgi?id=731200
test: add test for explicit-notify properties

https://bugzilla.gnome.org/show_bug.cgi?id=731200