Added "Auto-hide Panel" option to Panel menu

See feature request: https://github.com/linuxmint/Cinnamon/issues/3361

Before:
![panel context menu](https://cloud.githubusercontent.com/assets/597206/3594302/f5b06122-0c98-11e4-9aca-26e5fe3506d3.png)

After:
![panel context menu changed](https://cloud.githubusercontent.com/assets/597206/3594294/c1d85210-0c98-11e4-94b1-1f72ed82046b.png)