Fix source widget context menus

[d-chambers]

Summary

• keep the standard Orange widget context menu actions for source widgets
• append Set Active Source after the normal widget actions
• filter DerZug widget entry points to the DerZug distribution so third-party widgets do not pollute registry checks

Validation

• prek run ruff ruff-format --files src/derzug/views/orange.py tests/test_views/test_orange_qt.py src/derzug/utils/misc.py tests/test_utils/test_misc.py
• env MPLCONFIGDIR=/tmp/mpl pytest tests