Merge deprecated 'pythonalgorithms.directories' key with new key...

automatically and add old plugin location to base python.plugins key.
Avoids users seeing confusing warnings.
Refs #7103
Point ConfigDialog at user.python.plugins.directories instead of...

deprecated pythonalgorithms.directories key.
The other code changes simple update the variable names to say plugin
rather than algorithm.

Refs #7103
Merge branch 'feature/7103_migrate_pyalgs_prop_key' into develop