Meta files are not compatible between versions

[erlioniel]

Describe the bug
Metafiles generated by versions 1.0.0-1.0.1 are not compatible with the newest (1.0.2+) version in any way. Basically adding the newer version will nullify all settings. I think there should be a migration step that will detect the old meta format and will convert it to the new one.

However, during the investigation I figured out that the same behavior is not achievable by the newest versions of the importer, for example:

• Bundled tile grid doesn't work for me completely and misses the grid size parameter
• Bundled version of the plain file is not importing multiple sprites and because of that animation is not working.

I guess this issue should be handled when Bundled version of the importer will be stabilized and can be postponed until then.

To Reproduce
Steps to reproduce the behavior:

1. Create a file & import it using 1.0.1 version of the importer
2. Bump version of the importer to the newest one

Expected behavior
Metadata files should be migrated and users should be warned about dramatic functionality changes beforehand.