Divide windows into types

[jcreedcmu]

At least make a split between real documents and modeless about box.

Doing this requires making sure all of the following files, which refer to docTE, don't unconditionally assume that every app window has one:

• [x] view-rects.c
• [x] windows.c
• [x] menus.c
• [x] handlers.c
• [x] file-ops.c
• [x] scrolling.c