Verify workingness of all supported legacy serializers

[goblinJoel]

Some legacy serializers may have been removed or have lost save capability while retaining load. We should verify that the legacy serializers currently in the project work correctly for the following cases:

If a format supports both legacy save and load...

• TK3 save
• TK4 load
• TK4 save
• TK4 load again
• TK3 load again

If a format supports legacy load only...

• TK3 save
• TK4 load

An asset should contain correct information in all cases. For extra thoroughness points, it could be tested in the TK3 engine to ensure both engine and editor still read an asset correctly after TK4 has re-saved it.

It may also be useful to have an asset checklist showing current status (unsupported, untested, supported) for each asset for both legacy and JSON formats. Obviously, JSON is the must-have, legacy load is should-have, and legacy save is nice-to-have.

I apologize for my unfamiliarity with github workflows. If someone thinks this should be multiple issues or a milestone or something, I have no objection to them making that change.

[goblinJoel]

Lengthy related discussion at PR #7. Some of the legacy serializers may have been reimplemented with off-by-one issues with arrays of values, missing or extra nulls at the end of things, etc.

[goblinJoel]

It sounds like, while we will hopefully allow importing of old assets for any that haven't changed too drastically, we will not be supporting saving in the legacy formats. With the demise of the old site, need for creating files that TK3 can use is negligible.

May be best to move the legacy loading into a separate Import menu item instead of the main Open one. We can keep the file selector widget smarter that way and more clearly separate them.