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/imagecanvassource.js

@@ -1,6 +1,5 @@
 goog.provide('ol.source.ImageCanvas');
 
-goog.require('ol.CanvasFunctionType');
 goog.require('ol.ImageCanvas');
 goog.require('ol.extent');
 goog.require('ol.source.Image');