GSoC 2009: Widget API

(Its GSoC code time and we're starting our overhaul now! This is the first of many issues that will follow. All GSoC issues will be tagged as gsoc2009 and gsoc2009-lut4rp.)

A long time requested feature has been a way to change the voting widget. The only way to do that in 1.x was to modify the images and CSS. This is inspired from Fivestar (http://drupal.org/project/fivestar) which has a method for users to select different "star sets".

Our goals:
- Show a list of all available widget themes
- Allow users to choose different themes for every implementation (eg: nodes, comments)
- Allow each widget theme have its own style (up, up/down, down)

Implementation ideas:
- Base it on template files (implemented in UpDown - http://drupal.org/project/updown)
- keep widget engine in vote_up_down.widget.inc
- keep widgets in ./widgets/
- template_preprocess_* hooks are already in use in the 1.x branch.
- More ideas? :-)