msgstore: implement a uid-based architecture

Change the message store architecture from an index-based to a uid-based
one. Key advantage of this design approach is that no reselect mechanism
is required anymore since it comes with the design for free.

Fixes: https://todo.sr.ht/~rjarry/aerc/43
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Tested-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

commands/account/clear.go

@@ -51,8 +51,6 @@ func (Clear) Execute(aerc *widgets.Aerc, args []string) error {
 
 	if clearSelected {
 		defer store.Select(0)
-	} else {
-		store.SetReselect(store.Selected())
 	}
 	store.ApplyClear()
 	acct.SetStatus(statusline.SearchFilterClear())