Use properties, not get* methods, in events

Following today's hangout, I've applied the technique described in this [email to the Closure Compiler list](https://groups.google.com/d/msg/closure-compiler-discuss/PrzDGyxuh7U/Lt9Q32-UpNQJ) to prevent renaming of properties in events. This means that we can use properties instead of methods.

I've not converted all the events yet, but the technique seems to work in the cases I've tested so far. It will definitely need some thorough testing with the fully compiled ol3.js.

Note that this technique means that these properties are *never* renamed, even when the application and ol3 are compiled together. There is a solution to this.

Note also that it would be probably sensible to generate the externs file automatically, similarly to what we do with `objectliterals.jsdoc`. This might also make documenting events easier.

CC @pagameba @tschaub
Refs #1213