glib-compile-schemas: warn about bad dconf paths

For quite some time the recommended usage of GSettings and dconf has
been to use paths like /org/gnome/example/.  Use of /apps/ has spilled
over from GConf and is continuing to make its way into a number of
applications as they port.

glib-compile-schemas will now warn about these types of paths being
used.  This generates a lot of noise, but hopefully it will reduce the
number of ported applications making this mistake.