Split startDate property into today and displayedDate in calendar component

Basically splits the Calendar::m_startDate into 'today' and
'displayedDate', where displayedDate is the date that is displayed (it
controls the days model etc) and can be manipulated by the user by eg.
changing months in the plasmoid, and today is the current day, populated
by our dataengine (which means it auto-updates with no need for a
timer).

This allows for greater flexibility and things like "Go back to today"
when eg. the plasmoid is hidden or when the user have browsed too far in
the calendar and just wants to get back to today (the button to do that
pending).

Also this fixes a problem where the time dataengine is being polled
every 30secs for the clock and would reset the calendar view as the
startDate is currently bound to the dataengine and the view resets on
that change.

REVIEW: 118668