listview: Make inertness not depend on factory

listview: Make inertness not depend on factory

The inertness concept introduced in 62e9d1e470b assumed a listview was
inert when no factory was set. This has 2 problems:

1. columnview uses a listview without factories.

2. header factory being set but factory not being set technically makes
   the listiew inert - but should it?

So for now, make inertness only depend on visibility and root.
A side benefit is that this matches columnview semantics.