Support scrolling all scroll regions with the mouse wheel also while popup menus are open. Support overlapping scroll regions by always scrolling the top-most hovered scroll region on mouse wheel events.
The hot scroll region is now tracked separated by `CUi`, as tracking the IDs of all UI elements which are contained in scroll regions is not feasible. The separate active state for scroll regions is therefore unnecessary.
It's still necessary to disable `CListBox`es when popup menus are open, to ensure that only one list box consumes the key events.
Closes#8087. Supersedes #8090.
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