Rework the UI framework

Interface is now Driver, and Widget is now a set of interfaces with a
struct per widget type. This should make it easier to add other types.

internal/assetstore/menu.go

@@ -21,13 +21,29 @@ func (m *Menu) Records() []*menus.Record {
 
 func (m *Menu) Images(start, count int) ([]*ebiten.Image, error) {
 	out := make([]*ebiten.Image, count)
+
+	sprites, err := m.Sprites(start, count)
+	if err != nil {
+		return nil, err
+	}
+
+	for i, sprite := range sprites {
+		out[i] = sprite.Image
+	}
+
+	return out, nil
+}
+
+func (m *Menu) Sprites(start, count int) ([]*Sprite, error) {
+	out := make([]*Sprite, count)
+
 	for i := start; i < start+count; i++ {
 		sprite, err := m.Sprite(i)
 		if err != nil {
 			return nil, err
 		}
 
-		out[i-start] = sprite.Image
+		out[i-start] = sprite
 	}
 
 	return out, nil