refactor garmin_fs class

[tsteven4]

• use a Qt container for ilinks.
• let the container manage memory, taking advantage of copy on write.
• replace garmin_fs_alloc with new.
• make garmin_fs_t* constant where possible, eliminating the possibility of the ilinks list detaching.
• return an optional from garmin_fs_t::convert_category instead of returning status and passing a pointer to the output value.
• retire garmin_fs_convert_category. This was mostly duplicated between garmin_fs_t::convert_category and the caller.
• create garmin_fs_t::print_categories from the original in garmin_txt, and use it in both garmin_txt and the gpx writer.
• add opt_utc to garmin_txt reader (it was already in the garmin_txt writer)
• correct bugs in garmin_txt function strftime_to_timespec. %H was mistranslated, %p was missing, if the format code wasn't found a character from outside the buffer could be used. add warning when a conversion isn't found.
• add a test proving categories can go from gpx to garmin_txt and back. additionally the test exercises the ini file category support.
• improve debug message in convert_from_human_[time|date]_format.

[robertlipe]

Consistently good stuff.. %his is so much less fragile and easier to read.. Thanx.