Rework Button bootstrap theme

The first goal was to properly manage text overflow in all cases (text and/or icon). I had to update the widget to use one SPAN for both text + icon instead of 2. Icon is now part of a before pseudo element:
```js
.d-icon-cut:before {
	background-image: url('images/cut.gif');
	width: 16px;
	height: 16px;
}
```
Here is an overview of how it looks:
![screenshot from 2014-04-08 21 07 57](https://cloud.githubusercontent.com/assets/4415497/2647969/2ee25a9e-bf51-11e3-998d-59b8f00faf2c.png)