Add ATK window activation events for popups

In the accessibility tree, each popup menu and its submenus have a
native window. When using popup menus, the AtkWindow for the native
window of the active menu should always have the ATK 'active' property.
We listen to events associated with the lifetime active popup menus and
use them to properly manage the 'active' property on this constellation
of AtkWindows.

Bug: 895921
Bug: 896392
Change-Id: I7a5e148feb8486bab7984f19c7ee2532b1fd16f0
Reviewed-on: https://chromium-review.googlesource.com/c/1335586
Commit-Queue: Martin Robinson <mrobinson@igalia.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612770}
11 files changed