New attribute in level spec to define tileset - Backwards Compatibility

[autofly17]

(I am not a programmer, and this is my first-time using GitHub, so bear with me)

As an idea to preserve backwards compatibility with older/unmaintained contribute levels, I propose adding a new attribute to the level spec that defines whether the level uses the old (0.6.3) or new (Nightly) tilesets.

My design proposal goes like the follows:

• Add a newtiles bool, which we will include and set to #t for already-converted levels.
• Levels that don't have newtiles will default to the old tileset.
• Attempting to edit an old tiles level will create and edit an alias with newtiles set to #t (With the appropriate warning message displayed, of course).

However, there are a few drawbacks to my proposal that I'd also like to mention:

• The installation would have to come with the old tiles included, which would be an added 7.36 MB
• newtiles is not a forwards-compatible name; if we decide to do another tileset overhaul (unlikely), the naming convention may get awkward.

[Semphriss]

It is already planned to remap the new tile graphics to the old format.

[Semphriss]

Update: #2462 is now open, though still WIP. I'll post more upates there.