Badge the exception (not yet cached) instead of the near-universal case

GpxStore#save converts eagerly, so "cached" is true for nearly every
file in the tree already — badging that was just noise. Flipped to
PENDING_ICON, shown only for the rare case: a .gpx/.fit dropped
straight into data/gpx/ on the filesystem, bypassing the app, and not
yet opened.

Also add an explicit download button next to delete (DOWNLOAD_ICON) —
the file name itself has been a download link since the last commit,
but isn't obviously clickable at a glance.

public/css/style.css

@@ -379,6 +379,22 @@ html, body {
   height: 14px;
 }
 
+.gpx-tree__download {
+  flex-shrink: 0;
+  display: flex;
+  color: #999;
+  padding: 0 4px;
+}
+
+.gpx-tree__download svg {
+  width: 14px;
+  height: 14px;
+}
+
+.gpx-tree__download:hover {
+  color: #333;
+}
+
 .gpx-tree__delete {
   flex-shrink: 0;
   border: none;