Remove unnecessary duplicate code dealing with invoking window from wxOSX.

Remove unnecessary duplicate code dealing with invoking window from wxOSX.

This is roughly the same as r64127 for wxGTK but for wxOSX: don't duplicate
the functionality already present in the base class in Mac-specific way. Just
use wxMenu::GetWindow() instead of painstakingly propagating invoking window
changes via the entire menu hierarchy.

Also attach the root menu used in wxOSX to the menu bar to ensure that the
correct window can be found for all its menus.

Closes #11990.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64136 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775