removed a limitation in the dds texture loader.

The loader did not accept all textures
dds texture libraries added
add open and save support for dds textures in texture viewer
dialog for converting textures added
fixed some bugs in the DDS-Exporter
conversion now executed in a swing worker
rewritten dds support so that no external libraries are used.

the nvidia dds texture tools are used instead.
See https://developer.nvidia.com/legacy-texture-tools --> DDS Utilities
added a pre-order traversal of the scene graph.

Called depthLastTraversal to keep name coherent with depthFirstTraversal
fixed some missing change
changed depthLastTraversal to depthFirstTraversal with an extra flag
Merge branch 'extended-dds-support'
Merge branch 'pre-order-traversal'