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

* 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.

Add Evolution Data Server (EDS) calendar integration to the MATE panel
clock applet, most of it ported from gnome-panel's clock applet. This
allows users to view calendar events and tasks directly from the clock
popup calendar.

The calendar integration is disabled by default and can be enabled
with --enable-eds during configuration. When disabled, the clock
applet functions normally without any EDS dependencies.

Screen functions are replaced with their Display equivalents to remove
deprecation warnings. Display functions take into account the entire
display server connection, rather than a single physical monitor.

Before this commit, the Clock Applet's popup window would expand off the
screen if the user added more than a few locations to the Locations list.
In addition, the list of locations in the Clock Applet Preferences could get
extremely tall as well, and could even push the bottom part of the
Preferences dialog off the screen.

This commit adds (vertical) scrollbars to both widgets.

%l is an glibc extension for strftime, this extensions isn't supported
by musl libc.

Replace %l with %_I, which is functionaly equivalence.
%_I itself is also an extensions, however, %_I is supported by both
glibc, musl, FreeBSD and deriviates, AIX, and Solaris.

Fix: #1451