We currently sync the number of workspaces with the corresponding
preference. This is not really useful with GNOME Shell's dynamic
handling of workspaces, not least as the setting is effectively
ignored. Worse, it will trigger writes to dconf on login, slowing
down startup, so add a setting to indicate that workspaces are managed
dynamically and really ignore the num-workspaces setting when set.
https://bugzilla.gnome.org/show_bug.cgi?id=671568
Move preferences to GSettings, using mainly shared schemas from
gsettings-desktop-schemas.
Unlike GConf, GSettings support is not optional, as Gio is already
a hard dependency of GTK+.
https://bugzilla.gnome.org/show_bug.cgi?id=635378