Add GPX drag & drop, folder management, delete, and folder-first sort

Uploading is now drag & drop (or click to browse) instead of a form;
folders can be created any time, not just during upload; files can be
dragged onto a folder to move them, or dropped elsewhere in the tree
to move back to the top level; files and folders can be deleted. Both
drag sources (an OS file drag and dragging an existing tree entry)
land on the same drop targets, told apart by a custom drag payload
type versus the browser's own "Files" type.

Also sort folders before files in the tree (previously interleaved
alphabetically).

app.rb

@@ -63,5 +63,51 @@ module Maptool
         halt 422, json(error: "could not parse GPX file")
       end
     end
+
+    post "/api/gpx/folder" do
+      begin
+        GPX_STORE.create_folder(params[:path].to_s)
+      rescue GpxStore::InvalidPath
+        halt 400, json(error: "invalid path")
+      end
+
+      json tree: GPX_STORE.tree
+    end
+
+    delete "/api/gpx/file/*" do
+      begin
+        GPX_STORE.delete_file(params[:splat].first)
+      rescue GpxStore::InvalidPath
+        halt 400
+      rescue Errno::ENOENT
+        halt 404
+      end
+
+      json tree: GPX_STORE.tree
+    end
+
+    delete "/api/gpx/folder/*" do
+      begin
+        GPX_STORE.delete_folder(params[:splat].first)
+      rescue GpxStore::InvalidPath
+        halt 400
+      rescue Errno::ENOENT
+        halt 404
+      end
+
+      json tree: GPX_STORE.tree
+    end
+
+    post "/api/gpx/move" do
+      begin
+        path = GPX_STORE.move(from: params[:from].to_s, to_folder: params[:to_folder].to_s)
+      rescue GpxStore::InvalidPath
+        halt 400, json(error: "invalid path")
+      rescue Errno::ENOENT
+        halt 404
+      end
+
+      json path: path, tree: GPX_STORE.tree
+    end
   end
 end