Replaces duplicated scrolling code with re-usable functions.
Removes excessive boundary checking on scroll code, since that's done
in the scrollbar control anyways.
Properly separates repaint calls based on what has changed.
Send EN_UPDATE and EN_CHANGE at the right places.
Only call EnsureVisible on changes, not all repaints.

This avoids having to generate all the headers before make depend.

Also build the .c and .h files separately to allow parallel makes.