Fixed leader generation to be more conformant with common expectations.

Fixed leader generation to be more conformant with common expectations.
Everything assuming leaders automatically expand as much as possible
is broken now, leaders expand ONLY due to text justification.
Expanding multiple leaders on a line is simplistic.
Fixed leader-alignment="reference" to some extend. Leaders may be
still misaligned if text justification causes spaces to expand, i.e.
slack space was not taken completely by leaders.

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/branches/fop-0_20_2-maintain@195925 13f79535-47bb-0310-9956-ffa450edef68