Relax Unity menu interdiction hack to allow popup menus in modal dialogs.

Relax Unity menu interdiction hack to allow popup menus in modal dialogs.

The changes of r73007 fixed the problem with menus remaining active while a
modal dialog was shown (see #14823) but also disabled processing of the events
from any popup menus shown by the dialog itself. Reallow the latter while
still forbidding the former now by checking whether the window associated with
the menu is the dialog itself.

Closes #14961.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73365 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775