Make sure we update the calendar on date change

This introduces new property on MonthView - "today". This property is
updated by dataengine (or can be a timer too, but we already get signals
from dataengine every 30 seconds to update the clock) and is never
updated by the code itself.

Furthermore, the DayDelegate is now bound to it, so when "today"
changes, the selection rectangle in the calendar should also change.

And finally, the selected item in the grid is cleared if MonthView's
date property is cleared (which is second part of this patch, to the
applet itself).

REVIEW: 118277