instead of a simple bool, we now have five states: preview, active,
addon, obsolete, and ignore (the default). the default includes the
first three.

the CI system is expected to use --module-subset=all,-ignore to include
everything that is expected to build (in some configurations).

Change-Id: Ifb43412054a8e42db0425f24f8e53acfce363caa
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>