Propagate the event handling fixes to wxVarScrollHelperBase.

Propagate the event handling fixes to wxVarScrollHelperBase.

Merge the fixes to wxScrollHelperBase::ProcessEvent() of r64358, r64370,
r64464, r72939 and possibly a few more in wxVarScrollHelperBase to fix its
behaviour too, as it wasn't generating the correct events any longer.

Unfortunately the fix right now is to physically copy the code from one class
to the other. This should be avoided, of course, and a more in depth
refactoring should be done to move the code common to both classes into
wxAnyScrollHelperBase after 3.0 release. But for now continuing to duplicate
code is better than not having a working wxVarScrollHelperBase.

See #15357.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74814 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775