_frontend/widget.py: Render core messages more like other messages

In order to test when core activities occur by parsing the stderr
in tests, we should make the messages conform more.

At the same time, this restores alignment of columns in core
messages with the element processing related messages.

Also, _scheduler/scheduler.py is updated to make it's activity names
conform to the (current) 5 character limit for the sake of alignment.

The tests/frontend/logging.py test gets it's regex updated for
the log line it checks for in stderr.