Add option within OpenFixMap to turn the GPS functionality on and off

[rjw62]

I would like to suggest adding an option to turn off the GPS tracking from within the app.

Having the GPS on used quite a bit of battery life, and isn't necessary a lot of the time. Perhaps there could be an icon to toggle it on and off next to the cross-hair on the top right.

(Yes, you could turn the GPS off globally in the phone, but you may not want it off for other apps, so it would be more convenient to just have the option for OpenFixMap to not use the GPS unless you wanted it to.)