GUI: delay the GUI state saving a bit

When the user moves a slider, resizes a column or something, it's better to
delay the actual save operation a bit so that the changes are compressed. It
isn't that much useful to save each value in a range separately anyway.
GUI: remember column sizes automatically

This would get hit when the user resizes the window or any QSplitter, but it's a
bug that it wasn't saved automatically.

v3: Do not use any extra signals. Thanks to Thomas for review.

REVIEW: 113383