[+][BUG][ED] Improved handling of missing pieces

[WillLem]

SuperLemmix Editor fixes this. We now get a single popup (as opposed to many), and the level's missing pieces are listed in a clickable dialog.

The level is loaded as fully as it can be with all missing pieces deleted, and - if saved - is appended with "(Missing Pieces)" so that it doesn't overwrite the original.

I plan on making further improvements to this in due course. Placeholder images aren't really practical due to shape/size variables, but there may be a better way to handle the process which doesn't involve deleting the pieces.