* Fixed bug with row selection with an Enter key. (Row was selected
twice, if it was selected again in filtered dropdown)
* Fixed a nasty related bug with row filtering and multiple selection.
(Rows wasn't selected property with multiple selection after filtering.)
* Implemented selection with a spacebar.
* Replaced deprecated boolean checks, that already have a method
equivalent.
* replaced `if` - `else if` - `else if` - `...` with `switch`
Merge pull request #666 from enonic/CMS-3251

CMS-3251 ComboBox: Support for selecting multiple options using spacebar