Add :split and :vsplit commands, which split the message list view to
include a message viewer. Each command takes an int, or a delta value
("+1", "-1"). The int value is the resulting size of the message list,
and a new message viewer will be displayed below / to the right of the
message list. This viewer *does not* set seen flags.
Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>
