added IUnknown::getReferenceCount and IMeshCache::clearUnusedMeshes to flush unused meshes from the mesh cache.
Fixed CGUIModalScreen, which I broke with the IGUIElement changes. Also fixed a small bug with the tooltip appearing directly under the mouse cursor.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@760 dfc29bdd-3216-0410-991c-e03cc46cb475