prefs: Add dynamic-workspaces setting

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