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.h

@@ -45,8 +45,13 @@ class GPS : public PeriodicTask, public Observable
     /// Restart our lock attempt - try to get and broadcast a GPS reading ASAP
     void startLock();
 
+    /// Returns ture if we have acquired GPS lock.
+    bool hasLock() const { return hasValidLocation; }
+
   private:
     void readFromRTC();
+
+    bool hasValidLocation = false; // default to false, until we complete our first read
 };
 
 extern GPS gps;