CMS-3393 Image Picker - Add support for keyboard usage

Added support for the Tabulation and Enter, Backspace and Delete keys
for the selected options.
Added implementation of the focus and blur events.
Renamed removeCheckListener to unChecked.
Fixed wizard panel form icon from being stretched.
Merge pull request #719 from enonic/CMS-3393

CMS-3393 Image Picker - Add support for keyboard usage