Fix sizes of internal controls of AcquireImageControl

A Mono layout bug truncates the size of controls anchored right and
left inside AcquireImageControl.  This provides a workaround to fix
https://jira.sil.org/browse/BL-778 "[Linux] Image Toolbox only
displays 2 columns of AoR images (Windows does 3)".  The bug would
probably affect WeSay as well as Bloom.
So maybe add a comment explaining this?