jobqueue: simplify the signature of Job::factory() and Job::__construct()

jobqueue: simplify the signature of Job::factory() and Job::__construct()

Remove the $title argument from these methods to simplify subclasses that
do not have a meaningful title to use. The Job::getTitle() method can be
overriden by subclasses to return something meaningful.

The old call signature is still supported for backwards compatibility.
This will automatically determine what getTitle() returns as before.

Use "Blankpage" as the "not applicable" title for jobs instead of one
that looks like some error occured.

Change-Id: I3d5bd012d9cef1e7daaccfb0d5d319552eb89fb6