CMS-3102 Add button for opening the dropdown of a ComboBox

Added support for arrow disabling event, when maximum occurrences
reached.
Fixed dropdown arrow height. Made it autosizable.
Updated combobox input padding, so that the long text wont overlap the
dropdown arrow.
Merge pull request #622 from enonic/CMS-3102

CMS-3102 Add button for opening the dropdown of a ComboBox