[RFC PATCH] GPX layers should use the name from the file instead of `Downloaded GPX Data` whenever possible

[taylor.smock]

Right now it is not possible to set the name of a GPX layer on download. This means that GPX layers are commonly called Download GPX Data. It would be nice, if there is a name in the metadata, if JOSM could use that instead.

Use cases:
Tasking Managers could send an XML file that looks like this:

This XML file does not appear to have any style information associated with it. The document tree is shown below.
<gpx creator="HOT Tasking Manager" version="1.1">
  <metadata>
    <link href="https://github.com/hotosm/tasking-manager">
      <text>HOT Tasking Manager</text>
    </link>
    <time>2019-12-31T21:56:28.586152</time>
    <name>Tasking Manager: Project 1, Task 1</name>
  </metadata>
  <trk>
    <name>Task for project 1. Do not edit outside of this area!</name>
    <trkseg>
      <trkpt lat="39.070328" lon="-108.561121"/>
      <trkpt lat="39.070438" lon="-108.571025"/>
      <trkpt lat="39.077663" lon="-108.57099"/>
      <trkpt lat="39.077499" lon="-108.56091"/>
      <trkpt lat="39.070328" lon="-108.561121"/>
    </trkseg>
  </trk>
  <wpt lat="39.070328" lon="-108.561121"/>
  <wpt lat="39.070438" lon="-108.571025"/>
  <wpt lat="39.077663" lon="-108.57099"/>
  <wpt lat="39.077499" lon="-108.56091"/>
  <wpt lat="39.070328" lon="-108.561121"/>
</gpx>

Please note that the current version of the tasking manager (TM3) creates an empty layer with the desired name and then loads in the GPX file.

[taylor.smock]

Initial patch (no tests)

[taylor.smock]

As I was writing the test, I noticed that the default name comes from the file name.

What should be preferred? Name from file name, or name from the metadata? I (think) name from file name should be preferred, but it will make it harder to write a test case.

[taylor.smock]

Move newLayerName logic to getLayerName function. Also, remove a pointless call in GpxLayer (super(nameInFile); setName(name);, where the super call essentially sets the name is pointless).