Add button/dialog to customize text preview

[JoeStrout]

After #3, add a "..." button near the text preview. Clicking this should pop up some sort of dialog in which the user can type a new string to use as the preview text.

The nicest way to do this would be to grab the textField module out of the Inversion Institute project. It will need a little bit of cleanup to separate it from that project, but hopefully not too much. Note that the field in this case should use a standard font, like minimicro-pro-16, regardless of what font the user is editing.