Don't process events in ParseProjectJob::start()

From KJob::start() documentation:
Warning: Never implement any synchronous workload in this method.
This method should just trigger the job startup, not do any work itself.
It is expected to be non-blocking.

When a user exits KDevelop in the ParseProjectJob::start()'s nested
event loop, RunController may be destroyed in the time between
`job->start();` and `checkState();` statements in
RunController::registerJob(). The result is a segmentation fault in
RunController::checkState().

BUG: 427386
FIXED-IN: 5.6.1