Fix plugin loading for Linux (and presumably all other platforms as well)
Fix compilation of OSG example plugin
Separate default plugin paths for debug and release. Note that this is only important for plugins from 3rd party projects, since own plugins are loaded from the build directory, which should be separated anyway. However, install targets for plugins still have to be defined to target plugins/ or plugins/debug
Merge branch 'master' into navigation

Conflicts:
	source/examples/viewer-qt/main.cpp