Introduce preferences file

We now use a GSettings object with a custom backend that writes to a
configuration file, ~/.chimara. Right now there is only one example
setting, "flep". It's easy to bind the setting to a property of another
GObject, like the "active" property of a checkbox.

Still need to check how other GLib applications install the settings
file properly in their Makefile.am, and how to work with a default
configuration file such as /etc/chimara if the one in the home directory
is not present.