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

Updated `arrow down`. It will open the dropdown and navigate to the
first row, if necessary.
Updated `arrow up`. It will remove active row on press, if the current
active row is the first one.
Merge pull request #679 from enonic/CMS-3251

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