compositor: use g_slice allocator for compositor windows

On my system, 2-4% of the CPU time in xfwm4 is being spend in
find_cwindow_in_screeni() pulling in entries from the linked list on
a lightly updating desktop.

After this change, 25-50% of that CPU is gone, presumably because the
slice allocators have better locality of the list-entries.

Closes: #416