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/assetstore/menu.go

@@ -16,8 +16,8 @@ type Menu struct {
 }
 
 // FIXME: don't expose this
-func (m *Menu) Records() []*menus.Record {
-	return m.raw.Records
+func (m *Menu) Groups() []*menus.Group {
+	return m.raw.Groups
 }
 
 // FIXME: don't expose this