Use lowerCamelCase for the JSON API (fixes #1338)

Replace the current mix of UpperCamelCase und lowerCamelCase with
consistent lowerCamelCase keys for the JSON API. Also adapt the frontend
so it works with the changed API.

Attention: this will break existing consumers of the API.

gui/scripts/syncthing/app.js

@@ -78,7 +78,7 @@ function folderCompare(a, b) {
 function folderMap(l) {
     var m = {};
     l.forEach(function (r) {
-        m[r.ID] = r;
+        m[r.id] = r;
     });
     return m;
 }