Minor improvements to duplicating to new location

[marc-vdm]

Resolves the following bugs:

1. database of own activity was not always added to checked databases
2. better handling with multiple locations
3. replaced exchanges are now deleted from the new process instead of the old one (oops)
4. context menu option now respects read-only state of the database (is now disabled when database is readonly)

Improves the following:

• Can now choose RER and/or Europe without Switzerland as alternatives
• candidates are now found in both source database and dependent database, allowing users to create new supply chains and relink them on the fly.

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update the documentation, please follow the numpy style guide
• Categorize the PR by setting a good title and adding one of the labels:
  bug, feature, ui, change, documentation, breaking, ci
  as they show up in the changelog

[coveralls]

coverage: 50.059% (-0.1%) from 50.18%
when pulling 231f26c on marc-vdm:location_linking_bugs
into 015d6e7 on LCA-ActivityBrowser:master.

[github-actions]

This issue has been implemented in the new release of Activity Browser 🚀 (version 2.9.3), you can get the new version by updating Activity Browser.

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