DiscoverObject: Address quitting with concurrent KJobs
We have a mechanism that disables random eventloops (KJobs) from quitting Discover outside of the user's interaction (see QCoreApplication::setQuitLockEnabled in discover/main.cpp).
It so happens that we would at times have KJobs running also after Discover's window closes. See how on the reported bugs they often describe it happening early in the startup process.
This patch restores the Q*Application::quitLockEnabled property so that once Discover is with its task and we let go and QCoreApplication can let these jobs finish and eventually quit.
It changes the DummyBackend to issue a KJob at the start to be able to reproduce this problem and eventually its fix.