Pass spacing on FlexColumn to it's inner ColumnLayout

There's no way to set the inner column's spacing from outside the component, so it falls back to Qt's default of 5. This change makes it possible to set it to something more sensible by the application.

Please backport to KF5, since we will use this in Tokodon and we'll be under Qt5's grip for a little while longer. I would consider this a bugfix 😄