Fixed assert when the user stops the running list job.

Testcase: `dolphin /`, Ctrl+F, *.h, press stop button in status bar.
The code said "leave the job running, other views might be interested,
and the job is just like any other update job", but this use case shows
that this isn't wanted (and there are no update jobs for kio_filenamesearch).

Implementation details: jobs are now killed using kill() in order to
trigger the real implementation of slotResult/slotUpdateResult; added
a doKill for implementing it in CachedItemsJob. Less duplication overall.

Added unittest about deleting a KDirLister before it has time to emit items,
as found during dolphin startup.