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.

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

Mostly found by cppcheck.

origin commit was:
https://github.com/mate-desktop/mate-panel/commit/96c7ebc

This reverts commit 96c7ebc6dc9e8b8327db04a9570054ee78743353.

Mostly found by cppcheck.

There is no need to return -1 or 1 specifically, anything on the right
side of 0 is OK, so simplify using the usual `a - b` implementation for
such sort functions.

Note that the type is OK here, as the `struct tm` are ints, the same as
the function's return value.

* Clock applet/wayland: position calendar window same as in x11

- Wayland/clock: fix calendar window rendering when moved
Ensure the calendar window cannot be "stretched" across the screen
if the panel or the applet is moved while the calendar is showing