[✓][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.

[WillLem]

OK, we now have much better handling of missing pieces. The status bar informs the user that the level contains missing pieces, and offers 3 options (via the "lemming" button):



Show missing pieces - Opens a dialog with a list of the missing pieces
Keep missing pieces - Closes the status bar and returns session to normal (NOTE: the missing pieces will remain in the level file, and will be flagged as missing again next time the level is opened in the Editor)
Delete missing pieces - Removes the missing pieces from the level

So, importantly, the Editor no longer auto-deletes missing pieces and instead gives the user complete control over the session. N.B. It was also decided to remove the behaviour of auto-creating a "_MissingPieces" version of the level. If the user wishes to create a new save of the level with a different filename, they can do this manually.

I'm fairly happy with this as a final version of missing pieces handling. I'll keep the topic open for now, but this should be considered resolved.

Implemented in commit 0fe3481.