Feature: text-based folder icons in directory mode

[mantacid]

This is partially related to this issue, but I have since learned that not everyone is experiencing the same catastrophic performance drop as I am.

Still, the folder icons in directory mode should use the same method of rendering as the other file MIMEtypes. Benefits of implementing this include but are not limited to:

• improving performance (even if only by a little bit)
• visual consistency (by nature of the icon being part of the editor font)
• the offset created by the current folder image would be removed
• folder colors could be defined in the theme
• all directory contents would be displayed using the same function (afaik)

Let me know what you think.