If a task is still in the scheduler priority queue and its
priority is changed, it won't be moved into the correct queue.
We have to track the priority of the scheduled task, so we can
warn the developer to fix the code and actually handle re-start
correctly.

Since we don't want to traverse the whole Scheduler queues on
priority change (which sometimes get very long) to remove the
wrong data item, we'll just invalidate it, if a priority change
is detected.

This also reverts commit d24b264c ("vcl opengl: avoid task
priority warning on cursor blink"), which tried to avoid the
warning, which was just half right and independent of the broken
priority change handling.

LO doesn't change priorities of scheduled tasks normally, so
that bug didn't turn out to have much impact, I guess.

Change-Id: I6e46b518a7c3532047c619c013bd8597f73ed7a6
Reviewed-on: https://gerrit.libreoffice.org/69249
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>