Yet another fix to event propagation in scrolled windows.

Yet another fix to event propagation in scrolled windows.

Calling ProcessEventLocally() didn't work because the command events were not
propagated to the parent window any more, breaking a lot of things including
wxDataViewCtrl event generation in the generic version. So do restore
ProcessEvent() call but use it on the next handler (i.e. the window itself)
and not this one now. This still results in some duplicate calls but at least
the events should be passed everywhere where they are expected to arrive.

wxScrollHelperEvtHandler must die.

Closes #12078.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64370 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775