Bad icon theme mapping for 'pencil' and 'edit'

According to the current binding:

• the silk 'pencil' icon is bound to the 'pencil' name, but also to the fa/gl/el 'edit' icons. fa/gk/el have their own versions of a 'pencil' icon, so binding it to the edit icon is wrong.
• no silk icon is bound to the 'edit' name.

While I understand that, for backwards compatibility reasons, the 'pencil' name binds to 'edit' icons from fa/gk/el, because silk has no 'edit' icon, just a 'pencil' one, I think it is a bad practice and we should not be doing it.

What we should do is to clearly bind the name 'pencil' to both silk's 'pencil' and to fa/gk/el's 'pencil so that we have a consistent mapping and, at the same time, a good enough fallback for old apps using the new skin/theme.

We should also bind the 'edit' name to silk's 'pencil' so that new apps will fallback gracefully on older skins/themes.