Add MenuScene with New Level UI

* Debug mode: no longer enables the DebugOverlay (F3) by default, but
  does now insert the current FPS counter into the window title bar.
* ui.Frame: set a default "mostly transparent" BG color so the frame
  background doesn't render as white.
* Add the MenuScene which will house the game's main menus.
* The "New Level" menu is first to be added.
  * UI lets you pick Page Type and Wallpaper using radio buttons.
  * Page Type: Unbounded, Bounded (default), No Negative Space, Bordered
  * Fix bugs in uix.Canvas to fully support all these page types.

frame.go

@@ -21,6 +21,7 @@ func NewFrame(name string) *Frame {
 		packs:   map[Anchor][]packedWidget{},
 		widgets: []Widget{},
 	}
+	w.SetBackground(render.RGBA(1, 0, 0, 0)) // invisible default BG
 	w.IDFunc(func() string {
 		return fmt.Sprintf("Frame<%s>",
 			name,