Added letters to the preview template to provide better context for the icons

Adding letters allows you judge quickly whether you need to redo the
icon or not. Here's a screenshot of the outcome:

![fontcustom_glyphs_preview-2](https://f.cloud.github.com/assets/1181420/962741/3f5dbbd2-04f5-11e3-858e-42193bc1960a.jpg)