fix #49: make debug screen show real data

* Break out debug screen to a separate class and make it thread-safe.
* Break out power state to a separate class.
* Show battery voltage, charging & USB status on debug screen.
* Show GPS lock / no lock
* Fix an off-by-one that I introduced earlier in `drawRows`.

src/GPS.cpp

@@ -16,7 +16,6 @@ static uint32_t
     timeStartMsec; // Once we have a GPS lock, this is where we hold the initial msec clock that corresponds to that time
 static uint64_t zeroOffsetSecs; // GPS based time in secs since 1970 - only updated once on initial lock
 
-static bool hasValidLocation; // default to false, until we complete our first read
 static bool wantNewLocation = true;
 
 GPS::GPS() : PeriodicTask() {}