Refactor ObjectLoader to use Dicts correctly, rather than abusing their internal properties

The `ObjectLoader` currently takes an Object as input, despite actually working with `Dict`s internally. This means that at the (two) existing call-sites, we're passing in the "private" `Dict.map` property directly.

Doing this seems like an anti-pattern, and we could (and even should) simply provide the actual `Dict` when creating an `ObjectLoader` instance.
Accessing properties stored in the `Dict` is now done using the intended methods instead, in particular `getRaw` which (as the name suggests) doesn't do any de-referencing, thus maintaining the current functionality of the code.

The only functional change in this patch is that `ObjectLoader.load` will now ignore empty nodes, such that `ObjectLoader._walk` only needs to deal with nodes that are known to contain data. (This lets us skip, among other checks, meaningless `addChildren` function calls.)

src/core/annotation.js

@@ -390,9 +390,8 @@ var Annotation = (function AnnotationClosure() {
         if (!resources) {
           return;
         }
-        var objectLoader = new ObjectLoader(resources.map,
-                                            keys,
-                                            resources.xref);
+        let objectLoader = new ObjectLoader(resources, keys, resources.xref);
+
         return objectLoader.load().then(function() {
           return resources;
         });