model va: fix spelling in comment
gui fine alignment: do not update the fine alignment dwell time if other tab is selected

also ensure we only put valid values.
gui lens align: ensure the scale window of the SEM view is updated

the code was almost correct, but subscribing to a VA doesn't keep a ref to the function, so we need to explicitly do it.
gui canvas: correctly fit to content at the very beginning

Make sure that fit to content gets the actual center position even at the very beginning.
This fixes a bug in the lens alignment that made the optical image very small if it has already be moved in previous GUI runs.