Each tree entry now shows a small waypoint/track icon based on a
cheap substring check for <wpt>/<trk> (Maptool::Gpx.kinds) rather than
a full REXML parse, since re-parsing a several-thousand-waypoint file
on every tree fetch would make the panel noticeably slower to open.
Also fix a real drop-target bug: .gpx-tree had no flex:1, so a
short/empty tree left blank panel space below it that wasn't part of
the drag & drop target — dropping a file there silently did nothing.
randy
06ea8ea141