Added settings for custom screensaver

Screenshot:

![screenshot from 2014-07-17 23 07 28](https://cloud.githubusercontent.com/assets/1816681/3622611/7953c20c-0e39-11e4-9c72-02d01722a2a1.png)

Depends on https://github.com/linuxmint/cinnamon-desktop/pull/16