Set popup menu invoking window in wxWindowBase and not in all ports.

Set popup menu invoking window in wxWindowBase and not in all ports.

Don't duplicate the code for setting (and unsetting, which was forgotten by at
least wxGTK) the popup menu invoking window in all ports but do it in the base
class PopupMenu() itself.

Also add a helper wxMenuInvokingWindowSetter class which ensures that the
invoking window will be unset in any case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64143 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775