Don't write "Local GPX" in imagery_used changeset tag if that's not true

[jidanni]

Some trace ids should be recorded in changesets
openstreetmap/openstreetmap-website#1645 (comment)

[bhousel]

Right now the imagery_used tag always says "Local GPX", but instead it should probably write the name of the file.

This is because:

• it might not be local (i.e. passed in via url api)
• it might not even be a gpx, as we allow other types like .geojson and .kml now.

The code that does this is here:

iD/modules/renderer/background.js

Lines 92 to 95 in f7b7bd7

	var gpx = context.layers().layer('gpx');
	if (gpx && gpx.enabled() && gpx.hasGpx()) {
	imageryUsed.push('Local GPX');
	}

So, a better thing to do would be to add an accessor to that gpx layer, so this code can call something like gpx.getFilename().

[jidanni]

I will tell GitHub, that when a
"This was referenced on Nov 11, 2017" is added to a bug,
they really should notify all people subscribed.