garmin gpi reader and writer enhancements (#154)

garmin gpi reader and writer enhancements (#154)

* enhance garmin gpi reader and writer.
1. fix a few memory leaks.
2. properly encode category passed from user to writer.
3. add support for reading gpi files with dual language strings.
4. add support for code page 65001, i.e. utf8.
5. make sure writer embeds encoded string lengths instead of the lengths of source QStrings.
6. enhance test to cover all of the above.

* correct reference mode.

* fix garmin_gpi test for windows.

* rename garmin_gpi country code to language code.