visual-mode: deselect messages after performing command

In order to better align to vim functionality: deselect visual mode
selections after performing a command on the selection. This patch also
introduces a new command to allow for re-selecting (remarking) the
previous selection set so that commands can be chained together. The
deselection only applies to msg commands that *do not* move the message
from the store (those types of commands already deselect):
- read/unread
- flag/unflag
- modify-labels
- copy
- pipe

Previous usage to mark several messages as read and deselect all:
Vjjj:read<Enter>:unmark -a<Enter>

New usage, similar to vim:
Vjjj:read<Enter>

To chain a command together:
Vjjj:read<Enter>:remark<Enter>{next command}<Enter>

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

commands/msg/copy.go

@@ -59,6 +59,7 @@ func (Copy) Execute(aerc *widgets.Aerc, args []string) error {
 			switch msg := msg.(type) {
 			case *types.Done:
 				aerc.PushStatus("Messages copied.", 10*time.Second)
+				store.ClearVisualMark()
 			case *types.Error:
 				aerc.PushError(msg.Error.Error())
 			}