Use the Glib slice allocator for fixed-size objects

The slice allocator, available since GLib 2.10, is more
space-efficient than [g_]malloc.  Since BarnOwl is obviously at the
leading edge of space-efficient technology, this seems like a natural
fit.  Use it for every fixed-size object except
owl_viewwin_search_data (which would need an extra destroy_cbdata
function to g_slice_free it).

Signed-off-by: Anders Kaseorg <andersk@mit.edu>