fixes #16939 IE hangs when switching between multiple editors / refocusing browser window, the problem also happened on IE8 and IE11.
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 trunk.
InlineEditBox: destroy save/cancel button widgets, fixes #17748
test for BackgroundIframe on all browsers, refs #16268
Don't treat SPACE as the start of a keyboard search.
Add KeyNavContainer robot test.
Fixes #17766.
Fix menu positioning when opened via keyboard.  Fixes #17769.

Changes include:
  - make keydown handler process context-menu key in addition to shift-F10
  - different approach to avoid repeated contextmenu key events;
    10ms delay in _scheduleOpen() was too short