Update of MultipleSelectionCombo to use checkboxes removes scrolling

[kasemir]

In the context of #2474 the MultipleSelectionCombo was updated from a combo with drop-down that allows selecting multiple items to a widget that shows a list of checkboxes, again to allow selecting multiple items.

As a side effect, the list no longer 'scrolls'. In the above screenshot, there are many more entries in the list of logbooks, but the drop-down ends at the bottom of the screen with no way to 'scroll', or to use cursor keys to go further down.