[select2] Make it possible to clear the input of Select2Choice and Select2MultiChoice

Mail discussion: http://markmail.org/message/u2ccale6aubcmm5p
Demo app: https://github.com/solomax/WicketSelect2Clear
Related issues:
- https://github.com/ivaynberg/wicket-select2/issues/30
- https://github.com/ivaynberg/wicket-select2/issues/29
- https://github.com/ivaynberg/wicket-select2/pull/54