Wizard actually destroyed, any threads safely exited

[mrvisscher]

The database import wizard was not properly deleted at first, leading to multiple wizard instances reacting to signals, causing mayhem. The wizard is now properly closed, and threads are safely finished.

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.
• Link this PR to related issues by using closing keywords.
• Request a review from another developer.
• Check-out on Linux because of Make DatabaseImportWizard more secure #44
  • test:
    • Linux working
    • Linux not 2 wizards open at 1 time

[coveralls]

coverage: 50.932% (+0.05%) from 50.879%
when pulling 1e331f1 on mrvisscher:multi-wizard-fix
into 6931bad on LCA-ActivityBrowser:master.