garmin.js: Support multiple path and file types

[ypetya]

Not every garmin has a Garmin/GPX directory. My edge has a Garmin/Activities instead. The available path and purpose could be figured out from Garmin/GarminDevice.xml:

[salomvary]

Interesting, I did not know about GarminDevice.xml.

To keep things simple we can just scan Garmin/Activities in addition to Garmin/GPX and adjust the file extension filter to *.fit files.