Make it possible to override clean for a product

I build some external dependencies with qmake via qbs. This works fairly well when building as I know the output artifacts that I need (libraries), but it falls apart when doing clean/rebuild. The reason is that qbs have no notion of all the intermediate parts that qmake (or some other build tool) creates, and is thus hampered.

In my project, a good solution would be to add a clean property to product where I could run some specific clean commands (with qmake that would just be qmake clean) instead of (or probably together with) qbs clean, similar to Rule.prepare.