Return correct invoking window for submenus of a popup menu.

Return correct invoking window for submenus of a popup menu.

SetInvokingWindow() is only called for the top menu being popped up itself but
the invoking window should also be associated with its submenus.

Modify GetInvokingWindow() to return the parents invoking window for submenus.

This fixes a crash due to returning NULL from wxMenu::GetWindow() in wxMSW
owner-drawn code.

And it also makes redundant some code in wxUniversal wxMenu implementation
which can now simply use GetInvokingWindow() in all cases.

Closes #11957.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64104 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775