MetaDataStore drops database when it's deleted from the project

[mrvisscher]

The MetaDataStore will now check whether it's databases are still existing when a database is removed from the project.

• fixes deleting database does not remove it from metadata #1419

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update the documentation
  • For in-code documentation, please follow the numpy style guide.
  • For user documentation, please update the wiki in
    ./activity_browser/docs/wiki
• Update tests.
• Link this PR to related issues by using closing keywords.

If you have write access (otherwise a maintainer will do this for you):

• 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.
• Add a milestone to the PR (and related issues, if any) for the intended release.
• Request a review from another developer.

[coveralls]

coverage: 53.123% (+0.005%) from 53.118%
when pulling 62da4d3 on mrvisscher:metadata-sync-on-db-delete
into eeb1685 on LCA-ActivityBrowser:main.