Fixed notifications

Took me a long while to figure out the various reasons for things going wrong.. but figured it out in the end.
GTK demo is now truly asynchronous using the shortest possible route for file descriptor events.
Note it is not necessary at all to create a custom GSource, as g_unix_fd_add will create its own source
and maintain that for us.

Also note the client will only react to G_IO_PRI and G_IO_IN, but will terminate the source should we encounter G_IO_HUP (which is a registered interest in the source creation).

To confirm my changes, edit your buxton.conf as normal, launch bt-daemon and run two bxt_gtk_clients, they'll both react to changes on the base layer. Note I now explicitly use get_value_for_layer, because it wasn't used previously, and I discovered my user layer already had a test:test key overriding.. and I didn't see changes :)