Show waypoint photos from newer p4n2gpx exports

A newer p4n2gpx run (Ruby rewrite, ~/test/mapping/p4n2gpx) started
emitting one plain GPX <link href> per park4night listing photo,
which this app parsed but never displayed. Gpx.waypoint_photos
collects them into a photos property; the waypoint popup renders them
as a row of thumbnails above the description, each opening full-size
in a new tab. Absent on older exports and plain GPS waypoints, so the
row just doesn't render for those.

test/fixtures/sample_poi.gpx

@@ -20,4 +20,9 @@
       <locus:icon xmlns:locus="http://www.locusmap.eu">res/drawable-hdpi/ic_parking_small.png</locus:icon>
     </extensions>
   </wpt>
+  <wpt lat="48.103" lon="17.203">
+    <name>With photos</name>
+    <link href="https://cdn3.park4night.com/lieu/1_100/1_gd.jpg"><text>Photo</text></link>
+    <link href="https://cdn3.park4night.com/lieu/1_100/2_gd.jpg"><text>Photo</text></link>
+  </wpt>
 </gpx>