Enable mouse event processing for ListWidget adapter children

[whitebyte]

Fixes #538

Disclaimer:

1. I only checked it with mouse events, not keyboard events
2. Git history shows that this line was never uncommented, it was added in b07d0aa already commented out. The commit was introduced in Tooltips are not shown #303. There, @buggins states: "No mouse events are being passed to items in list widget. List handles them all.". I examined widget.d and see that it's only partially true. ListWidget indeed manages its children hover/selected state, but that's it. There are use cases when it's not enough, e.g. I want to open a popup menu by right-clicking a ListWidget child added through adapter.

Uncommenting the line fixes the issue for me and looks like a straingtforward thing to do, but then again I don't know what was @buggins ' motivation and design goals there. So, proceed with caution