GObject: warn on use of deprecated properties

By default G_PARAM_DEPRECATED means absolutely nothing.  We only emit a
warning if G_ENABLE_DIAGNOSTIC is set to '1' and then, only on sets.

Turn the logic on its head: emit the warning by default, unless
G_ENABLE_DIAGNOSTIC is set to 0.  In order to avoid a torrent of output, only
emit a warning once per property name.

https://bugzilla.gnome.org/show_bug.cgi?id=732184
Revert "gsettings: remove long-deprecated "schema" property"

This reverts commit cf9b162e0d0defaff2ad5c85aeaf7af0899ad22f.

It turns out that there are still a very large number of users of this
API.

https://bugzilla.gnome.org/show_bug.cgi?id=732102