Major tidy up of Scroll.c to add foundations for flatscroll bars

[György 'Nog' Jeney]

> I have used this code to implement flat scroll bars in two ways however
> I run into problems and I am looking for advice.

Hi,

I was thinking about implementing flatscrollbars but never got round to
implement the Flatscrollbars itself.  I thought about doig it like this:
Subclass the window with flatscrollbars, with the Subclassing functions
that I have implemented recently. Implement GetScrollBarInfo and use it to
get all of the scrollbar properties like thump position and size, arrow
sizes etc.  Store an internal list of the System metricies (just like
native) and then draw the control. All of the functions that pertain to
flat scroll bars will just call their user counterparts and the same goes
for messages.

nog.