[WIP] Support multiple @find-by annotations

Still open tasks but basic support for multiple ```@find-by```. Still requiring some new unit tests and ```ElementCollection``` see #48 for more details.

Closes #13
This is old code. We don't guess which collection class to use based on which given element class in `@var` annotation. If user wants to use a collection then he specifies it's class (e.g. `RadioGroup`) directly in `@var` annotation.

If you see `@var Checkbox[]` then regular proxy can deal with that automatically being the collection itself.

Getters and setters for that `collectionClass` need to go away as well, since nobody uses them right now anyway.