* cppheader.xsl had initially been written under the false assumption that a
missing oor:nillable attribute defaults to "false" instead of "true".  That has
been fixed.
* As a result, many places that use the new simplified officecfg/*.hxx headers
broke as they did not expect value types to be wrapped boost::optional.  To keep
the code simple, I decided to change all occurrences in
officecfg/registry/schema/ of properties that specify a default <value> and do
not explicitly specify oor:nillable="true" to oor:nillable="false".  Strictly
speaking, this is an incompatible change, but in many cases it should be what
was intended, anyway.
* Some places that use the new simplified officecfg/*.hxx headers still had to
be adapted to boost::optional wrapping.
* This showed that unotools/configuration.hxx did not yet work for those wrapped
properties and needed fixing, too.