Make every operator bool() explicit

operator bool() is a trap, but with explicit it's far safer,
and we can use that now.

Change-Id: I4e58631c94e87c00256c3ab3cff4fd2c5f632713
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>