Add multi-image support to widgets.

Closes #579 

![multi-image-select](https://cloud.githubusercontent.com/assets/96776/5194030/da48fe6c-74bc-11e4-8c27-30d7f183a347.png)

![multi-image-select](https://cloud.githubusercontent.com/assets/96776/5194029/da47d276-74bc-11e4-8296-a223981ddf5f.gif)