The old add-ons UI had a "Refresh" operator that would re-scan
the file-system and show any changes.
Add an equivalent operator that works for both extensions and legacy
add-ons and add it to the extension menu.
This is useful for a few situations:
- Refresh based on changes to "System" repositories which are read-only
from Blender and intended to be manipulated outside of Blender.
- Anyone managing their own "User" repository.
- For developers to create extensions, where re-starting to see changes
to meta-data is inconvenient.
- Troubleshooting problems parsing extension meta-data as problems are
reported to the operator.
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