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