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

• [x] Keep pull requests small so they can be easily reviewed.
• [x] Update the documentation, please follow the numpy style guide.
• [x] 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.
• [x] Request a review from another developer.
• [x] Check-out on Linux because of #44
  • test:
  • Linux working
  • Linux not 2 wizards open at 1 time

[coveralls]

coverage: 50.932% (+0.05%) from 50.879% when pulling 1e331f103b46e3b47a283b6a3d373e2c94002de7 on mrvisscher:multi-wizard-fix into 6931bad51a55d68bdbffafcfa2f5d2ae9afa139e on LCA-ActivityBrowser:master.