Calendar: Show multi-day event continuation

Currently, multi-day events are shown as individual appointments on each day.
This patch ellipsizes multi-day events to indicate continuation on the prior
or following day (or other time-period.)

The time label spot is now replaced by a box layout that contains the prefix
ellipsis label, the time label and the postfix ellipsis label.
In order to keep the alignment, ellipses are merely invisible (zero opacity)
when hidden.

The ellipses are styled using the events-day-time-ellipses class which, by
default, take the color of the event text.

When RTL is used, the box contents are adjusted accordingly (clutter does that
for us).

An event spanning three days now displays "...All Day..." in the calendar on
the second day.

https://bugzilla.gnome.org/show_bug.cgi?id=727302
Calendar: sort multi-day events by ending day/time

With commit dc6a60dde, the calendar displays the ending day and time of a
continuing multi-day event on its ending day. This results in the list not
appearing to be sorted. This patch sorts the list according to the displayed
day/time.

With the two appointments
Thursday 0800-1000 Foo, and Wednesday 0900-Friday 1200 Bar and today being
Monday, the rest of the week list currently displays:
F ...1200 Bar
T    0800 Foo
With this patch, the displaying order is switched because Friday comes after
Thursday.

https://bugzilla.gnome.org/show_bug.cgi?id=727302