Changed clear display icon in gnome-calculator

Old
![screenshot from 2014-03-20 13 52 41](https://f.cloud.github.com/assets/1816681/2476851/3d7e9500-b069-11e3-8f48-1bc0de330f2c.png)
New
![screenshot from 2014-03-20 13 50 40](https://f.cloud.github.com/assets/1816681/2476852/487f7e56-b069-11e3-807a-3212a89f2dca.png)