Phase out old typewriter mode implementatin for ST3
Rough-and-ready implementation using on_text_command (ST3 only)

- Pythonesque implementation of Typing Mode
- Disable mouse clicks & drags in Scrolling Mode

Suggested by [bizoo](https://github.com/bizoo) at the [ST Forum](http://www.sublimetext.com/forum/viewtopic.php?f=5&t=13012&sid=9e72fb150b1d2847509a034f468170bd&start=10#p54139)