js/ui/calendar.js: Generic calendar widget
tests/interactive/calendar.js: Basic test of the calendar
js/ui/panel.js: Add a pop-down from the clock that shows a
calendar widget. The pop-down is not menu-like to allow the user to
interact with an application while looking at the calendar.
gnome-shell.css: Add theming for calendar, calendar popup, and for
buttons on the panel
https://bugzilla.gnome.org/show_bug.cgi?id=596432
