Make the calendar reactive and handle scroll events to change the month.
(GtkCalendar and hence the old gnome-panel calendar supported this and
it is apparently a handy way to flip through months.)
The padding is moved from the CalenderPopup to the Calendar so that the
scroll region extends all the way to the edge of the popup.
https://bugzilla.gnome.org/show_bug.cgi?id=596432
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
