fixes #16939 IE hangs when switching between multiple editors / refocusing browser window, the problem also happened on IE8 and IE11.

(cherry picked from commit 95cc48933bef6ed4217a527dae4eef7a83f71059)
Updating metadata for 1.8.6
Updating source version to 1.8.7-pre
Remove evt.preventDefault() calls on dropdown mouse events.

The evt.preventDefault() on mousedown breaks ComboBox on iOS, by cancelling the mouseup
and click events, thus preventing selection of an option.

According to the comment, the evt.preventDefault() call on mousedown was to
"prevent focus shift on list scrollbar press".
But I still lost focus when clicking the scrollbar, at least on IE8.
Plus which, even if clicking the scrollbar loses focus, it doesn't seem like a problem.

There were also evt.preventDefault() calls on the other events (mouseup, mouseover, etc.) but I
don't see any reason for them.

Fixes #17752 on 1.8 branch (available from 1.8.6).
(cherry picked from commit 11074d6f8003459f73a169e6144659175bb3313e)

Conflicts:
	form/_ListBase.js
	form/_ListMouseMixin.js