Fix mousewheel scroll stop

This gets rid of the complex `#onMouseWheelEntry` behavior that has accrued and had slow and weird behavior. The desired wheel behavior is the following:

- The world zooms on CMD+wheel
- Morphs that are not clipped or have nothing to scroll do nothing but can implement the `#onMouseWheel` event handler to do custom stuff -- This means that those morphs don't intercept /stop panning of the world
- In active windows, if a scrollable/clipped morph is scrolled, world panning should be inhibited. This is useful because when working e.g. in the SCB and scrolling one of the lists it is not desired to accidentally scroll up/down the whole world when "overscrolling" the list.