Show and hide the inactive device buttons based on new count property

The old code was incorrect, calling rowCount as though it was a property, but it was instead a function, so it didn't update as expected.

This commit adds a count property that notifies of changes as expected to the abstract model as well as the filter model. Now the "Show inactive devices" buttons should be properly hidden by people whose audio setup has no hidden devices.

...But please test. :)