qt: fix right click on title bar opens context menu for the view delegate beneath with csd

We need to take exclusive grab here, otherwise handlers beneath that take exclusive grab may act on press.

This is the case when media library view items go under the title bar. Unfortunately in controls we have to use TapHandler.ReleaseWithinBounds as gesture policy for the tap handlers due to a bug present in Qt 6.2.

By the way, I also realized that with Qt 6.2 dragging the window once makes it impossible to drag every other time. I'm not sure if there is a trivial fix for that, but I have not spent a lot of time on it considering it does not occur with Qt 6.8.

Request review @chub.