Improved GPS Settings app and GPS service (#222)

- Fixes and improvements to `GpsSettings` app, `GpsDevice` and `GpsService`
- Implemented location/GPS statusbar icon
- Added app icon
- Added support for other GPS models (based on Meshtastic code)

TactilityCore/Include/Tactility/StringUtils.h

@@ -58,6 +58,11 @@ std::basic_string<T> lowercase(const std::basic_string<T>& input) {
     return std::move(output);
 }
 
+/**
+ * @return true when input only has hex characters: [a-z], [A-Z], [0-9]
+ */
+bool isAsciiHexString(const std::string& input);
+
 /**
  * @return the first part of a file name right up (and excluding) the first period character.
  */