terminal: View --> Guide doesn't show the guide when the terminal is not the main file type

[williamstein]

1. Open a file foo.py
2. Split the frame and make one frame a terminal
3. Select View --> Guide
4. The guide frame does not appear

The above works fine if the file is foo.term.

The problem is that the guide is a different type of frame that isn't configured at all for .py files (say).

How to fix:

• bad hack: in every place where we add "terminal" as an allowed frame type when registering an editor, also add the guide frame type. This is bad since it is confusing and clutters the UI.
• better hack: somehow just allow this guide to be opened anyways if terminal is there, but don't list it explicitly in any menus.

[williamstein]

Regarding fault: this bug was introduced by me when switching to the "new" frame system a long time ago...