feat(datepicker): make widget accessible

Tests and some polishing is missing, but I think this is one of the most accessible datepicker's out there! :dancers: 

Added:
 * Keyboard navigation in calendar and Escape on popup.
 * Focus handling when closing/open popup.
 * WAI-ARIA roles

Breaking changes:
 * Looping modes is removed. Ie from max mode you don't move to min mode.
 * For popup, open on focus is removed, but open with down arrow is added.