For now, I was only testing writing a file to the SD card from the FreeRTOS
apps. It doesn't yet work.

All other apps use the same button to exit, so the USB mass storage app was
changed in kind.

The setting_apply() and setting_get() functions were added, to make it easier to
work with the settings structs. Also, to make the GMT offset setting work with
these two functions, the setting was split into two settings, now of type
setting_t.

This app is not yet properly working, it doesn't always properly set time.
To be checked out exactly why.

Previously, this function was returning a human-friendly version of the
time received from GPS. This was now optimized to make it easire to operate
with other programming structures (e.g., struct tm). This modification has the
added benefit of returning the exact values received via the GPRMC NMEA
sentence.

Two more events were added to the event type (event.h)
- GPS_TICK -- tick from GPS background task
- GPS_OFF  -- to signal the status bar that the GPS was turned off,
so it can remove the icon

The status bar and GPS coordinate app were changed to react to the
proper events, i.e., both events above in the case of the status bar
and the GPS_TICK event in the case of the GPS coordinate app.

Tested with the GPS coordinate style in gpscoord.c

Not yet tested, hence the new branch