Set active_by_default to true when migrating if is_active is missing

[soininen]

When migrating, set all active_by_defaults to true unless is_active (or equivalent) has been defined. This mimics the pre-0.8 behavior where all classes were visible by default.

Fixes #spine-tools/Spine-Toolbox#2611

Checklist before merging

• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.22%. Comparing base (9415645) to head (8580b06).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #370 +/- ## ======================================== Coverage 78.21% 78.22% ======================================== Files 75 75 Lines 9433 9428 -5 Branches 1998 1995 -3 ======================================== - Hits 7378 7375 -3 + Misses 1726 1725 -1 + Partials 329 328 -1 ```

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