Move top-level typedefs to typedefs.js

With one exception, those directly in the `ol` namespace can be moved without problem.
Some left-over goog.requires are also removed.

src/ol/source/imagearcgisrestsource.js

@@ -4,7 +4,6 @@ goog.require('goog.asserts');
 goog.require('goog.uri.utils');
 goog.require('ol');
 goog.require('ol.Image');
-goog.require('ol.ImageLoadFunctionType');
 goog.require('ol.events');
 goog.require('ol.events.EventType');
 goog.require('ol.extent');