ui: add :split and :vsplit view options

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>

commands/account/split.go

@@ -0,0 +1,63 @@
+package account
+
+import (
+	"errors"
+	"strconv"
+	"strings"
+
+	"git.sr.ht/~rjarry/aerc/widgets"
+)
+
+type Split struct{}
+
+func init() {
+	register(Split{})
+}
+
+func (Split) Aliases() []string {
+	return []string{"split", "vsplit"}
+}
+
+func (Split) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (Split) Execute(aerc *widgets.Aerc, args []string) error {
+	if len(args) > 2 {
+		return errors.New("Usage: [v]split n")
+	}
+	acct := aerc.SelectedAccount()
+	if acct == nil {
+		return errors.New("No account selected")
+	}
+	n := 0
+	var err error
+	if len(args) > 1 {
+		delta := false
+		if strings.HasPrefix(args[1], "+") || strings.HasPrefix(args[1], "-") {
+			delta = true
+		}
+		n, err = strconv.Atoi(args[1])
+		if err != nil {
+			return errors.New("Usage: [v]split n")
+		}
+		if delta {
+			n = acct.SplitSize() + n
+			// Maintain split direction when using deltas
+			if acct.SplitSize() > 0 {
+				args[0] = acct.SplitDirection()
+			}
+		}
+	}
+	if n == acct.SplitSize() {
+		// Repeated commands of the same size have the effect of
+		// toggling the split
+		n = 0
+	}
+	if args[0] == "split" {
+		acct.Split(n)
+		return nil
+	}
+	acct.Vsplit(n)
+	return nil
+}