"Add widget button" looks crap and is innaccessible

[maxime-rainville]

The buttons to add new widget to a blog post are inaccessible ... you can't tab to them and they don't provide any indication that you can interact with them.

Here's the underlying HTML. <h3 title="Click to add this widget">Blog Tags</h3>

Also they look crap. Not sure if it's the retina display, but the image looks jittered. Let's maybe use a font-icon instead of an image.

[maxime-rainville]

@silverstripeux ^