Add tab completion to textinputs

This adds tab completion to textinput components. They can be configured
with a completion function. This function is called when the user
presses <tab>. The first completion is initially shown to the user
inserted into the text. Repeated presses of <tab> or <backtab> cycle
through the completions list. The completions list is invalidated when
any other non-tab-like key is pressed.

Also changed is some logic for current completion generation so that
all available commands are returned when <tab> is pressed with no
current text and similarly for arguments of commands.

commands/ct.go

@@ -19,6 +19,9 @@ func (_ ChangeTab) Aliases() []string {
 }
 
 func (_ ChangeTab) Complete(aerc *widgets.Aerc, args []string) []string {
+	if len(args) == 0 {
+		return aerc.TabNames()
+	}
 	out := make([]string, 0)
 	for _, tab := range aerc.TabNames() {
 		if strings.HasPrefix(tab, args[0]) {