Add "retina" graphics for high-dpi screens

Firstly if core devs aren’t interested in merging this sort of change, feel free to close this without guilt: I’m stuck in a holiday lodge due to the weather so am just passing the time!

This is mostly getting stuff set up ready for others to add updated graphics later - I don’t know where the icons came from so can’t source high-res copies of them! If someone can source them, or let me know where they came from, I’m happy to keep going through and adding them myself.

So far I’ve added high-res images for:

- `spinner.gif`
- `logo_small.png`
- `sprites-32x32/logout.png`
- `sprites-32x32/dialog-close.png`
- `sprites-32x32/dialog-close-over.png`

A little before/after example:

![1](https://cloud.githubusercontent.com/assets/1655548/4024126/ff71a41a-2bb9-11e4-9478-e1e204ec0d18.png)
![2](https://cloud.githubusercontent.com/assets/1655548/4024128/031fd082-2bba-11e4-8ac9-aa88ade75693.png)