Change fontsizeselect menu so it previews the font-size values

This pull request changes the styling of the list items in the fontsizeselect dropdown menu, so they preview the font-size which is to be selected, as shown in:

![fontsizeselect_preview](https://cloud.githubusercontent.com/assets/946583/3351189/53b14efa-f9fa-11e3-87e0-2053f13ca2ac.png)