by Davo Smith.
The code for turning editing on all occurs from about line 137 of course/view.php ( https://github.com/moodle/moodle/blob/master/course/view.php#L137 )
For editing to always be on you will need to set $USER->editing = 1 (but only after the 'user_allowed_editing' check).
Note, that there is a plan to tidy up the interface and remove the large number of icons, which would mean that editing could always be on for teachers. For the moment, I think it would be very messy to have it on all the time - too much clutter on the screen, when you don't need it.