ContextBrowserView: Fix deadlock

This happens everytime the context browser view is open at startup, and
KDevelop's background parser starts jobs.

Arguably, we should never block indefinitely in a non-user initiated event (here:
QWidget::showEvent). So let's give up after a timeout.

Differential Revision: https://phabricator.kde.org/D866
CCBUG: 355100