Add optional GPS shutdown timeout

[Martin Kolman]

• in some situations, it might be useful to turn off the GPS after a given timeout to save battery
  • this might be combined with some other rules
• when all the unneeded timers are removed, this might enable modRana to run in background for long periods of time without munching the battery flat

• or just leave the GPS in "standby" or check every n minutes, when the screen is turned off for a longer period of time, when this is enabled