qt: better looking trayicon

Fixes https://github.com/bitcoin/bitcoin/issues/3886 by letting qt do the downscaling from the original 256x256 bitmap.

Before/after on Ubuntu and Windows:
![qttray](https://cloud.githubusercontent.com/assets/8353616/3879362/ecc5b406-2177-11e4-936f-db4b27dd3add.png)