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/extent.js

@@ -1,4 +1,3 @@
-goog.provide('ol.Extent');
 goog.provide('ol.extent');
 goog.provide('ol.extent.Corner');
 goog.provide('ol.extent.Relationship');
@@ -6,14 +5,6 @@ goog.provide('ol.extent.Relationship');
 goog.require('goog.asserts');
 
 
-/**
- * An array of numbers representing an extent: `[minx, miny, maxx, maxy]`.
- * @typedef {Array.<number>}
- * @api stable
- */
-ol.Extent;
-
-
 /**
  * Extent corner.
  * @enum {string}