SvxBackgroundColorItem inherits from SvxColorItem and for backwards
compatibility with the StarOffice 5 binary file format (yes, really)
writes/reads only rgb and not the transparency value, so copying and pasting
text from a sidebar comment in writer to itself or another one results in a
black character background as the default COL_AUTO turns into black

Change-Id: I18b5105dd8e060b9e49dda6026e26d3a0f00d8f5