CMS-3103 Add support for selecting multiple options when ComboBox is open

Forwarded multipleSelections setting from ComboBox to DropdownGrid;
Added multiselect support;
Added Apply button to apply multiselect;
Fixed Dropdown Combobox position and width.
CMS-3103 Add support for selecting multiple options when ComboBox is open

Renamed apply button name from `multipleHandler` to
`applySelectionsButton`.
Merge pull request #630 from enonic/CMS-3103

CMS-3103 Add support for selecting multiple options when ComboBox is open