Display visible GPS satellites

This is a suggestion for a partly fix of #973. I put the "/" to the strings.xml for RTL compatibility.
![gps](https://cloud.githubusercontent.com/assets/3845150/4710017/c9f7c3e2-58a4-11e4-93d0-e31f8c27b999.png)

GPS needs at least 4 satellites to compute x, y, z and time, you can find this on [Wikipedia](http://en.wikipedia.org/wiki/Global_Positioning_System#Navigation_equations) and other sources.
I can revert this rather unrelated change here.
P.S.: apparently, it is not sufficient to check this value on Android. The first few answers to this [stackoverflow question](http://stackoverflow.com/questions/2021176/how-can-i-check-the-current-status-of-the-gps-receiver) are really interesting. According to the second one (and some other sources), there is a `android.location.GPS_FIX_CHANGE` intent (which I could not find in the API).