Allow dialogues to be hidden or shown

To do this, MENU and SUBMENU are split into two types (at last), and
a Widget type is introduced. This should allow lots of code to be
removed at some point.

internal/ui/dialogues.go

@@ -0,0 +1,30 @@
+package ui
+
+import (
+	"fmt"
+)
+
+func (d *Driver) Dialogues() []string {
+	out := make([]string, len(d.dialogues))
+
+	for i, dialogue := range d.dialogues {
+		out[i] = dialogue.Locator
+	}
+
+	return out
+}
+
+func (d *Driver) ShowDialogue(locator string) error {
+	for _, dialogue := range d.dialogues {
+		if dialogue.Locator == locator {
+			d.activeDialogue = dialogue
+
+			return nil
+		}
+	}
+	return fmt.Errorf("Couldn't find dialogue %v", locator)
+}
+
+func (d *Driver) HideDialogue() {
+	d.activeDialogue = nil
+}