Button size

This fixes a bug where image-only buttons would always be slightly too wide, meaning you couldn't butt them up nicely alongside each other. I originally needed this for https://github.com/ImageEngine/gaffer/pull/928, which we closed so I could work on the underpinnings of the SceneInspector first. Now I'm reimplementing #928 I don't need this fix but it's definitely worth merging in its own right.

This does slightly affect all layouts using Buttons, so @paulondc will need to review and see when is a good time for the merge from his point of view. There's no hurry from my point of view, but it'd be nice to get it merged before it gets horribly out of date.