The fix is twofold:
1. retrieve transparency from colour in SvxBackgroundColorItem
(add QueryValue, PutValue methods, use additional memberID to
retrieve alpha channel as a bool property)
2. add CharBackTransparent bool property to Draw [text] shapes,
xmloff needs it to be able to output 'transparent' string instead of
'#XXYYZZ' colour code in ODF format

Change-Id: Id830bd18fd6da5d9a46e2a94aa254d5d2a6b7ebb
Reviewed-on: https://gerrit.libreoffice.org/16182Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>