Add isGPSDetected() capability probe to SensorManager

Lets applications know whether a GPS was actually detected
(EnvironmentSensorManager reports its gps_detected state), e.g. to
hide GPS-dependent UI when no receiver is attached.

src/helpers/SensorManager.h

@@ -23,6 +23,7 @@ public:
   virtual const char* getSettingValue(int i) const { return NULL; }
   virtual bool setSettingValue(const char* name, const char* value) { return false; }
   virtual LocationProvider* getLocationProvider() { return NULL; }
+  virtual bool isGPSDetected() const { return false; }
 
   // Helper functions to manage setting by keys (useful in many places ...)
   const char* getSettingByKey(const char* key) {