Modify Mousewheel event to zoom text with Ctrl.

This pull request adjusts font size by using Ctrl key and the mouse scroll wheel.

This behaviour is familiar and native across most editors/IDEs.

I have previously created a package to implement this functionality (https://atom.io/packages/wheel-zoom), and it was commented that it could be introduced into the core (https://discuss.atom.io/t/use-ctrl-mouse-wheel-to-increase-decrease-text-size/11916).

First pull request, please be nice. :smile: 
The `decreaseFontSize` method is now on atom.workspace so please call it there instead.