Feature/progress bar

Again nothing very fancy, just a progress bar widget. Perhaps we could allow to display the percent value with a checkbox later.
![progressbar_widget](https://f.cloud.github.com/assets/24973/2493180/781e218e-b281-11e3-931f-5bb7d11058a2.png)
I hope to be able to create more sophisticated widgets (like a browser) later.