to make it work after the year 2038

See https://en.wikipedia.org/wiki/Year_2038_problem

This patch was done while reviewing potential year-2038 issues in openSUSE.

This allows users to open the clock/calendar popup through DBus, which
means we could configure a keybinding for it.

The Appointments and Tasks expanders in the calendar popup were
incorrectly linked to the show-calendar-events and show-tasks gsettings.
This caused collapsing the section to toggle the preference, making the
entire section disappear on next popup.

Fixed by adding a new expand-calendar-events and expand-tasks gsettings
keys to track the UI state separately from whether the section is
visible or not.

Evolution tracks a color for each calendar. We can fetch this to
differentiate events from multiple calendars in the popup.

This also marks the current event in bold and scrolls the window to make
it visible to the user on load.

* clock: Store calendar client in ClockData

This simplifies the evolution calendar client architecture and makes the
popup show up faster, since we don't need to create a new one every time
we open it.

* clock: Fix calendar data refresh when opening popup

The calendar popup was not showing recently added Evolution
events/tasks. Added a refresh timeout call every time the calendar popup
opens. That way the popup opens immediately and the data is refresh
after a small delay.

A previous overzealous cleanup left some hanging screen variables and
one instance of an uninitialized screen being used.

Provide a fallback preferences path in case the applet doesn't provide
one. This happens when running outside the actual panel, like in the
case of mate-panel-test-applets.

We remove a bunch of functions, since they are already handled by
libecal, and add correction to times based on the difference between the
event's own timezone and our current local timezone.