Import and display heart rate from GPX files

[Karry]

Shocker is requesting to add support for heart rate extension in GPX files. See forum thread.

GPX syntax:

<gpx xmlns="http://www.topografix.com/GPX/1/1" version="1.1" creator="Sports Tracker" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  xsi:schemaLocation="http://www.topografix.com/GPX/1/1  http://www.topografix.com/GPX/1/1/gpx.xsd http://www.garmin.com/xmlschemas/GpxExtensions/v3 http://www.garmin.com/xmlschemas/GpxExtensionsv3.xsd http://www.garmin.com/xmlschemas/TrackPointExtension/v1  http://www.garmin.com/xmlschemas/TrackPointExtensionv1.xsd"  xmlns:gpxtpx="http://www.garmin.com/xmlschemas/TrackPointExtension/v1"  xmlns:gpxx="http://www.garmin.com/xmlschemas/GpxExtensions/v3">
...
<trkpt lat="50.070905" lon="18.420838">
<ele>339.0</ele>
<desc>Speed 13.2 km/h Distance 0.01 km</desc>
<time>2022-07-09T09:41:23.35</time>
<name>2</name>
<sym>Dot</sym>
<extensions><gpxtpx:TrackPointExtension><gpxtpx:hr>122</gpxtpx:hr></gpxtpx:TrackPointExtension></extensions>
</trkpt>
...