[UIFe/FFe] Unity Control Center: add interface scaling options to Display panel

[Marco Trevisan]

Hi,

We would like to add to Unity Control center some options to improve the
user control over the new HiDPI scaling features of unity, so that it
would be possible to define how to scale the UI when in multi-monitor
mode.

This change will introduce 5 new strings to unity control center.
See more at: https://bugs.launchpad.net/unity-control-center/+bug/1297053

Cheers,
Marco