Get rid of stability annotations and document stability with api

This change adds a stability value to the api annotation, with
'experimental' as default value.

enum, typedef and event annotations are never exportable, but
api annotations are needed there to make them appear in the
docs.

Nested typedefs are no longer inlined recursively, because the
resulting tables get too wide with the current template.

externs/readme.md

@@ -13,7 +13,10 @@ For events, we make properties available to the application. Other than methods,
 /** @interface */
 oli.MapBrowserEvent;
 
-/** @type {ol.Coordinate} */
+/**
+ * @type {ol.Coordinate}
+ * @todo api
+ */
 oli.MapBrowserEvent.prototype.coordinate;
 ```
 In the source file (`src/ol/MapBrowserEvent.js`), the class needs to implement this interface:
@@ -29,13 +32,13 @@ ol.MapBrowserEvent = function(type, map, browserEvent, opt_frameState) {
 
   /**
    * @type {ol.Coordinate}
-   * @todo stability experimental
    */
   this.coordinate = map.getEventCoordinate(this.originalEvent);
 
   // ...
 
 };
+```
 
 ### Override methods in custom classes
 
@@ -69,7 +72,7 @@ ol.control.Control = function(options) {
 /**
  * Application subclasses may override this.
  * @param {ol.Map} map Map.
- * @todo stability experimental
+ * @todo api
  */
 ol.control.Control.prototype.setMap = function(map) {
   // ...
@@ -83,7 +86,7 @@ Object literals cannot be exported like classes. To make sure that their propert
 /**
  * @typedef {{element: (Element|undefined),
  *     target: (Element|string|undefined)}}
- * @todo stability experimental
+ * @todo api
  */
 olx.control.ControlOptions;
 
@@ -103,6 +106,7 @@ olx.control.ControlOptions.prototype.target;
 ```
 In the source code, the name used for the typedef is used as type whenever this object literal is expected:
 ```js
+/**
  * ...
  * @param {olx.control.ControlOptions} options Control options.
  */