qt: allow seek bar tool tip to go beyond the boundaries of interface window with Qt 6.8

Qt 6.8 brings this possibility, but I'm not sure if we should unconditionally do it. Showing and hiding a window is more expensive than showing an item in the scene graph, and I'm not convinced about the fading animation that some platforms apply when it is in a separate window. The main advantage of this is to be able to go beyond the interface boundaries, like VLC 3.

Alternative is adding an option for it.

Opinion @chub?