LinePlacement: 'center' and 'wordWise' extension

Additional deegree specific extentions added to LinePlacement.

This pull request is the same as https://github.com/deegree/deegree3/pull/219 which was reverted because of a build failure caused by a missing '-' character  in the sphinx source. 

compare:
https://github.com/IDgis/deegree3/commit/c059985272f06f7660af9e1457a5eec721c5637e
https://github.com/IDgis/deegree3/commit/83babfc08bec99fa1d9cf7a12f43bd9deb5cf290