Prevent QWebEngine from overriding signal handlers

Summary:
- The documentation view may use QWebEngine, which is based on
  Chromium, which installs its own signals handlers. This breaks
  the use of KCrash.
- Overriding the signal handlers can be prevented by setting an
  environment variable before the use, just like in D16004.

Test Plan:
- open a documentation page in KDevelop
- kill SIGABRT KDevelop -> DrKonqi shows up

Reviewers: #kdevelop, apol

Reviewed By: #kdevelop, apol

Subscribers: kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D16188