wxMenuBarManager::InstallMenuBarForWindow no longer looks for a menubar

attached to the parent of a window without one.  Instead, it is now up to
the GetAppMenuBar() function to do so.  The new implementation in
wxTopLevelWindow does just that.  The wxFrame implementation now calls
the base class version if it does not have a menubar.  Also, it is now
invalid to call the function with a NULL window (it is internal anyway).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@24424 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775