ArrayBox: Fix issues with auto-updating height
ArrayBox: Fix issues with auto-updating height

This fixes the following:
* vertical scrollbar displaying and immediately disappearing when adding
 new elements up to maixmum height
* calling `set_values` resulting in each item sometimes having incorrect
 height (e.g. 1 pixel)

khalim19/gimp-plugin-export-layers,khalim19/gimp-plugin-export-layers