These are not build system issues and deserve their own category.

This has the side effect of avoiding them to get cleared when e.g.
reparsing Qbs files.

Change-Id: Ib949504533f7f15a560796bea91cae329f19a20b
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>