Removing deprecations, dependency bumps, and some bug fixes

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

Recent versions of libwnck introduced a WnckHandler object to be used as
the main entry point into the library.

There are no distros (that I could find) that ship both an old
libwnck<3.32.0 with a new MATE>=1.28. Dropping this should be safe and
removes complexity.

Since all of these were already wrapped inside HAVE_X11, it should not
affect Wayland at all.

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.

All applet menus already use theme icon names directly, making this
legacy stock icon code obsolete. Removing it eliminates all the icon
factory deprecation warnings.

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.

Also replace gtk_dialog_add_button with panel_dialog_add_button to
ensure mnemonics and icons are preserved.

Since the icon loader is no longer present, there is no need to track
the icon theme when the applet is realized. This is will all be taken
care of by libwnck.