Added region-body_display-align to demonstrate the default behaviour for adjustable...

Added region-body_display-align to demonstrate the default behaviour for adjustable spaces when display-align is set to "auto".
Reactivated proprietary display-align="distribute" so a block-container can cause its content to be "justified" in the reference area if it's large enough and the adjustable elements (min/opt/max) allow for it. I may be misusing the original intended behaviour of "distribute" but for block-container this seemed suitable since it's destined to produce only one viewport/reference pair if possible. Test added that demonstrates the proprietary value on block-container. Note: on normal region-body elements, display-align="distribute" has currently no effect since the code did not evolve with the changes in the layout engine.

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@412735 13f79535-47bb-0310-9956-ffa450edef68