Better fit for slider outside.

Modified the slider so that the core should go all of the way through.
Marked the top of the piece slightly to make assembly easier.
Modify inside slider the same as outside slider.
Correct outside slider through-hole.

The new version resulted in an outside slider with a solid piece through
the hole. Extend the cut to make certain it stays.
Add support for generating an "assembled" STL.

This STL serves as documentation for how the parts should fit together.
Update documentation to support changes.
Move the 'top of slider' marks.

For some reason, the little cylinder on the outside slider caused a
glitch in slic3r that created a filled layer all the way across when
the little cylinder marker was on the lip. Moved to top without
side-effects. Made the same change to the inside slider, because it was
invisible on the original.