feat #994 MultiAutoComplete: Highlighted mode interactions

This pull request allows users to do the following:
1) When the user left-clicks on an item (that is not already highlighted),
it becomes highlighted (if any other was already highlighted, it becomes
de-highlighted)
2) When the user left-clicks on an already highlighted item, the latter
goes into edit-mode.
3) When an item is highlighted, pressing the backspace key removes the
item and the previous one becomes highlighted. If there is no previous
one, the following one becomes highlighted
4) When an item is highlighted, pressing the delete key removes the item
and the following one becomes highlighted

Close #994.