Make GcalMultiChoice and GcalDateChooser widgets accessible
A significant portion of the behavior was guided by the "Date Picker Spin Button Example" page in the ARIA Authoring Practices Guide (APG): https://www.w3.org/WAI/ARIA/apg/patterns/spinbutton/examples/datepicker-spinbuttons/
Related: #1036
Depends/Based on !605 (closed)
Edited by Hari Rana