Fix: allow the import of gpx files that use displayScale as Float xml node

[allandiego]

Fix the import of some gpx files that uses Float instead Double node on XProperties

Example of gpx xml part that was throwing error

<Bar id="29">
  <Clef>G2</Clef>
  <Voices>26 -1 -1 -1</Voices>
  <XProperties>
    <XProperty id="1124139520">
      <Float>1.07818</Float>
    </XProperty>
  </XProperties>
</Bar>

Issues

I was trying to import a gpx file with an error, tracing the error I found that was due to a child node from XProperties that the importer only looks for the Double node, but I found out in gpx that it can also be Float

Proposed changes

Look for both nodes types (Double and Float) to get the bar scale value

Checklist

• I consent that this change becomes part of alphaTab under it's current or any future open source license
• Changes are implemented
• Existing builds tests pass
• New tests were added

Further details

• This is a breaking change
• This change will require update of the documentation/website

[Danielku15]

Thanks for your contribution.