Increase / Decrease Font Size with the Mouse Wheel

This should complete the tasks in this [Trello Card](https://trello.com/card/2-increase-decrease-code-font-size/4f90a6d98f77505d7940ce88/462).

Unfortunately chrome doesn't trigger a mouse wheel event when control is pressed, so I had to use Shift + Mouse Wheel Scroll.

Updated: Since #3300 was merged, this request now only has the font size adjustments using the mouse wheel.