Workspace images

1) Generalizes WSS markup functions to IO actions
2) add Image widget to Workspace buttons, and include the hbox for more general control
3) add imageSelector to allow endusers to write their own image selection functions
![taffybar](https://f.cloud.github.com/assets/592062/2054920/308f98b6-8abd-11e3-866d-632eb4b3cae2.png)

this is a breaking change and it doesnt provide an elegant ootb solution for images. however, folks who dont want images dont have to do anything, and folks who do can write an imageSelector. if someone feels ambitious, a default imageSelector could be written to give a decent ootb experience.

i dont expect you to merge this, but it would be really great if you did, or impld something like it, not least because i could stop maintaining a fork.