qt: fix content getting exposed in header in `TableViewExt`

This is because clipping is not used here, so unfortunately if header is not inline and the view is interactive the content is exposed in the header.

We should use clipping here, for both viewport and event delivery (similar to play queue). Qt's ListView.OverlayHeader and ListView.PullBackHeader are practically useless, because unless the header/footer is placed outside the view, with these positioning we can not use clipping, and we need to handle the events manually in header and footer to not reach to the content.

I have put a note regarding this situation.

Regression since !7022 (merged).

Request review @chub.