Fix AB crash after two calculations

[mrvisscher]

Fixes an issue where the AB crashes after you've done two LCA calculations and then start another QThread. By supplying parents to QtWebEngine objects the issue is fixed, probably because they are now properly cleaned up after their parent widget has been closed.

Checklist

• [x] Keep pull requests small so they can be easily reviewed.
• [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.
• [x] Add a milestone to the PR for the intended release.
• [x] Request a review from another developer.

[coveralls]

coverage: 50.688%. remained the same when pulling 098f6a96c9ffa58d6d4deb810083dedd19e817d5 on mrvisscher:sankey-thread-crash-fix into 30a81c0220c523e029ae05c99cd9b6e874aa88c8 on LCA-ActivityBrowser:master.