No reference to internal types in externs file

Currently we have things like like in the generated externs file (types.js):

/**
 * @type {ol.control.AttributionOptions|undefined}
 */
olx.control.DefaultsOptionsExtern.prototype.attributionOptions;

It doesn't make sense to have external object properties whose types are
internal (ol.control.AttributionOptions in the above example).

With this commit, the generate-exports.py script generates this:

/**
 * @type {olx.control.AttributionOptionsExtern|undefined}
 */
olx.control.DefaultsOptionsExtern.prototype.attributionOptions;
Add export annotation @exportFunction

This commit adds a new export annotation: @exportFunction. @exportFunction is to be used to declare functions that take an options object literal as their first arguments. @exportFunction is to regular functions as @exportClass is to constructors.

The usage of @exportFunction is as follows:

@exportFunction ol.control.defaults ol.control.DefaultsOptions ol.Collection

Note: the Function#export method uses a recursive function. This is to handle nested options.
Use per-animation function modules

Currently the animation functions share the same module, namely ol.animation. We do differently for ol.control.defaults and ol.interaction.defaults, with ol.control.defaults and ol.interaction.defaults module. This commit replaces the ol.animation module by four modules, one for each animation function.

This makes things more consistent, and will make it possible to use the @exportFunction annotation for the exporting of the animation functions.
Make ol.Collection#extend return this

This to be able to do this:

new ol.Map({
  controls: ol.control.defaults().extend([new MyControl()]),
  ...
})
Export ol.Collection#extend
Use ol.Collection#extend in examples

This commit changes the examples to using ol.control.defaults().extend to extend the collection of controls. This is in preparation for a future commit that will remove the 2nd argument to ol.control.defaults. The same is done for ol.interaction.defaults.
Remove 2nd argument to ol.control.defaults
Remove 2nd arg to ol.interaction.defaults
Use @exportFunction to export ol.control.defaults
Use @exportFunction to export ol.interaction.defaults
Use @exportFunction to export animation functions