#25661 Switched to the new dark theme

Switches the photoshop engine to use the new dark theme introduced in https://github.com/shotgunsoftware/tk-core/pull/80

Also updates the application icon to use the engine icon - previously it was using a `resources/app.png` icon that didn't exist.

I have already done a quick QA on macosx, here's a screenshot of what it looks like:

![screen shot 2014-06-16 at 07 20 01](https://cloud.githubusercontent.com/assets/337710/3283807/05ac829a-f51f-11e3-97c3-7de52f3426d4.png)