Fix ol externs definitions

* Namespaces must be defined;
* Interfaces must be initialized.
Fix namespacing convention
Add 'tooltip'-tag to KML example.

Fixes #2708.
Merge pull request #2784 from marcjansen/tag-kml-example

Add 'tooltip'-tag to KML example.
Use valid mediatype for GPX export.
Merge pull request #2792 from pgiraud/gpx_export_chrome

Use valid mediatype for GPX export.
Merge pull request #2795 from gingerik/externs

Externs
Use goog.* types in externs

To have an externs file as accurate as possible, we also need to use
types that are provided by the Closure library.