Don't use invoking window in wxGTK wxMenuBar implementation.

Don't use invoking window in wxGTK wxMenuBar implementation.

wxGTK wxMenuBar used its own SetInvokingWindow/UnsetInvokingWindow() and
related functions instead of reusing the base class Attach/Detach() which
exist for exactly the same purpose. This resulted in unnecessary code
duplication and confusion and, since the changes of r64104, resulted in
asserts due to use of SetInvokingWindow() for non-popup menus.

Fix this by removing the wxGTK-specific functions and doing the work they used
to do in (now overridden) Attach() and Detach(). Also call Attach/Detach()
instead of these functions from wxGTK wxFrame and wxMDIParentFrame code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64127 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775