* fix#1538 log scrolls automatically
adds:
- when user scrolls, position is kept no matter how many new entries
arrive
- when user scrolls up to last entry, autoscroll is enabled again
does not add buttons to jump up/down
see #1538
* move scroll-test out of on_mouse_wheel
* determine diff_index with loop
* remove check at move_towards yoffset
* use while loop instead of repeat loop
* remove meaningless setter
* remove stray var
Luke aka SwissalpSandtakase1121