Only refresh menu when new puzzle created

Only refresh the puzzle menu after closing the
editor if a new puzzle has been saved.
Remove unused code

Remove unecessary code to expose puzzleId as a snippet
on PuzzleMenuIconView - this property is never used as a
binding.
Put label on create new icon

Place a label reading 'Create new puzzle' on the
icon in the menu for creating a new puzzle. This icon
receives a nil string for its textfield binding, use
a custom value transformer to convert that nil to
the correct string.