kwinrules: Use a list of desktop UUIDs to set and store the rules setting

Port the RuleSettings and KCM to store and use a list of virtual desktop
 UUIDs, instead of the previous x11 positional id, continuing the work on
This allows to set a rule with several desktops on Wayland.

On X11 it has no visible change for the user, but internally it uses the
more modern concept, helping to simplify the related code.

The relevant key on kwinrulesrc changes from `desktop` to `desktops`.
A kconf_update script handles the migration.