Better support for SVG slider handles.

* Scale WSliderComposed handle to the width or height of the widget.
* Update Deere slider handles to look more like classic Deere -- still
  missing the dropshadow.

Raster handles are usually custom-built to be the same width/height as
the slider so this should not affect most existing skins.
Add slider backgrounds modeled after classic Deere.
Merge pull request #423 from rryan/slider

Better support for SVG slider handles.