[Feature] add HTML capabilities to composer labels

Hi !

This adds a "render as html" checkbox to composer labels.

I think this features is a little bit overlapping with the html composer item, but :
1) unifying the HTML tool in the normal label seems logical: it still is a label, only a formatted one;
2) it is different in that you can use expressions;
3) it would be possible to implement html composer's item features (display an external page) in the new label tool.

This allows to make some advanced composition. For instance, you could display comma separated values as a table by replacing the comma by &lt;/td>&lt;td>.

![htmlFeature](https://f.cloud.github.com/assets/1894106/107835/d9229348-6a39-11e2-9291-d3f237a3ee97.png)