Updated toolbar icons, added Retina versions and original file

I designed the updated toolbar icons that appear in the preferences window before, and I've gone back and updated them.

There are minor tweaks which you can see in the differences/changes pane; I've also added Retina versions of all the icons; and the original file with the vector shapes has been added (zipped) (I used [Sketch](http://bohemiancoding.com/sketch/) to make the icons.)