1.12.5 BLOX.BDropDown: widget protocol
- Answer the rectangle in which the list widget will pop-up.
If possible, this is situated below the drop-down widget's
bottom side, but if the screen space there is not enough
it could be above the drop-down widget's above side. If
there is no screen space above as well, we pick the side
where we can offer the greatest number of lines in the
- Force the pop-up list widget to be visible.
- Answer whether the pop-up widget is visible
- Toggle the visibility of the pop-up widget.
- Unmap the pop-up widget from the screen, transfer its selected
item to the always visible text widget, and generate a callback.