Load MathCAT relative to the app directory (#19230)
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### Link to issue number:
Split from #19227.
### Summary of the issue:
MathCAT is loaded from `sys.prefix`, which does not reliably point to
the MathCAT directory on source copies.
### Description of how this pull request fixes the issue:
The MathCAT path is calculated relative to `globalVars.appDir`.
### Testing strategy:
Verified that MathCAT is usable on source copies.
### Known issues with pull request:
None known
### Code Review Checklist:
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- [n/a] Documentation: (out-of-scope, see #19227)
- Change log entry
- User Documentation
- Developer / Technical Documentation
- Context sensitive help for GUI changes
- [x] Testing:
- Unit tests
- System (end to end) tests
- Manual testing
- [x] UX of all users considered:
- Speech
- Braille
- Low Vision
- Different web browsers
- Localization in other languages / culture than English
- [x] API is compatible with existing add-ons.
- [x] Security precautions taken.
