Added warning dialogue to ActivityDelete action

[mrvisscher]

The ActivityDelete action now asks for a confirmation from the user.

• Closes Add confirmation dialog for deleting activity #1207

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update the documentation, please follow the numpy style guide.
• Update tests.
• 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.
• Link this PR to related issues by using closing keywords.
• Add a milestone to the PR (and related issues, if any) for the intended release.
• Request a review from another developer.

[coveralls]

coverage: 54.364% (+0.04%) from 54.328%
when pulling 455930d on mrvisscher:activity-delete-confirm
into 54d6a9c on LCA-ActivityBrowser:main.