Replace current terrain manager with Ogre terrain component

by Samuel Kogler for WorldForge

Ember currently uses PagingLandscapeSceneManager 2(PLSM2), a plugin for the Ogre 3D rendering engine which can display a Terrain by replacing the default Ogre scene manager. PLSM2 is no longer being developed and its design makes Ember inflexible. Therefore, it is desirable to switch to the new Ogre::Terrain component which is part of core Ogre and can be used independent of the SceneManager implementation.