A little more V, a little less M in the text editor

Requires PR #7109

- fill out basic menu items
- Add SaveWidget, copied from Notebook
- use config API to remember selections
- allow manual selection of CodeMirror mode

I tried to avoid making any style changes beyond what was necessary to get it working.

menus:

- File
  - New
  - Save
  - Rename
- Edit
  - Find
  - Find & Replace
  - Change keyMap (default, sublime, emacs, vim)
- View
  - Toggle Line Numbers
- Mode
  - listing of CodeMirror modes