Add sorting functionality

There is a command and config option. The criteria are a list of the
sort criterion and each can be individually reversed.

This only includes support for sorting in the maildir backend currently.
The other backends are not supported in this patch.

commands/account/sort.go

@@ -0,0 +1,85 @@
+package account
+
+import (
+	"errors"
+	"strings"
+
+	"git.sr.ht/~sircmpwn/aerc/lib/sort"
+	"git.sr.ht/~sircmpwn/aerc/widgets"
+)
+
+type Sort struct{}
+
+func init() {
+	register(Sort{})
+}
+
+func (Sort) Aliases() []string {
+	return []string{"sort"}
+}
+
+func (Sort) Complete(aerc *widgets.Aerc, args []string) []string {
+	supportedCriteria := []string{
+		"arrival",
+		"cc",
+		"date",
+		"from",
+		"read",
+		"size",
+		"subject",
+		"to",
+	}
+	if len(args) == 0 {
+		return supportedCriteria
+	}
+	last := args[len(args)-1]
+	var completions []string
+	currentPrefix := strings.Join(args, " ") + " "
+	// if there is a completed criteria then suggest all again or an option
+	for _, criteria := range append(supportedCriteria, "-r") {
+		if criteria == last {
+			for _, criteria := range supportedCriteria {
+				completions = append(completions, currentPrefix+criteria)
+			}
+			return completions
+		}
+	}
+
+	currentPrefix = strings.Join(args[:len(args)-1], " ")
+	if len(args) > 1 {
+		currentPrefix += " "
+	}
+	// last was beginning an option
+	if last == "-" {
+		return []string{currentPrefix + "-r"}
+	}
+	// the last item is not complete
+	for _, criteria := range supportedCriteria {
+		if strings.HasPrefix(criteria, last) {
+			completions = append(completions, currentPrefix+criteria)
+		}
+	}
+	return completions
+}
+
+func (Sort) Execute(aerc *widgets.Aerc, args []string) error {
+	acct := aerc.SelectedAccount()
+	if acct == nil {
+		return errors.New("No account selected.")
+	}
+	store := acct.Store()
+	if store == nil {
+		return errors.New("Messages still loading.")
+	}
+
+	sortCriteria, err := sort.GetSortCriteria(args[1:])
+	if err != nil {
+		return err
+	}
+
+	aerc.SetStatus("Sorting")
+	store.Sort(sortCriteria, func() {
+		aerc.SetStatus("Sorting complete")
+	})
+	return nil
+}