Bug 735895 - Precision Conversion "Dithering" dialog

Disable the convert precision dialog's dithering controls when
converting to higher bit depths o to anything > 16 bit.

Make sure the disabled dithering widgets always says "None", which
implies duplicating the bit depth checking logic in both the dialog
and its callback, in order to protect the values in GimpDialogConfig
from being overwritten by NONE.