Modified ViewerProperties to allow setting near and far plane via one function only.

Would lead to inconsistencies if they are updated one after the other (e.g. setting a larger near plane before far is increased will lead to a temporary near>far)

Improved detection of wrong near and far planes in the core.

closes #1368

git-svn-id: http://www.openflipper.org/svnrepo/OpenFlipper/branches/Free@16072 383ad7c9-94d9-4d36-a494-682f7c89f535