This change causes the time elapsed since the last periodic refocus
("Refocus every X minutes" in the capture tab) to be kept between
sequences. This enforces the periodicity of the refocus procedure.

When a sequence provides a different periodicity than the previous, the
refocus procedure executes at the specified different periodicity. The
initial delay before refocus in the new sequence thus is measured from
the last refocus time, instead of the start of the new sequence. It can
happen that the new sequence will cause a refocus immediately (in any
case after the first capture) if there was no or a large periodicity
in the previous sequence compared to the new.

Warning, as before, in order to use the periodic refocus, a preliminary
autofocus must be done, either manually in the focus tab or
automatically through the scheduler step "Focus".

A simple test is provided as "Tests/capture/jobs_using_refocus.esl". To
use that test, copy "Tests/capture/2x10x20s_refocus1min.esq" to
"/var/tmp/kstars_tests".

The test vector is made of three schedule jobs on Kocab, using a
sequence causing a periodic refocus every minute. The first job is
configuring a Focus step to avoid the capture tab failing to focus.