Ensure that sizeHint is 0 when sidebar is not visible
By default, Kate has an invisible sidebar on the right but it still takes a few pixel. This is particularly relevant since with plasma/breeze!368 (merged) the white space taken increase from 1px to 3px.
I'm not sure if this is the right way to do it and I find it weird that the minimumSize of an invisible item is not already by default equal to QSize(0, 0)
Before:
After: