Removed texComplete and added more general progressLevel

- progressLevel indicates how much of the data has been loaded
-- progressLevel == 0 => Still loading referered data elements
-- progressLevel == 1 => Still loading textures
-- progressLevel == Infinity => everything has been loaded
- This is currently supported for all data container
- Assets and model follow next
- In the future we can assign a "loadLevel" e.g. to data elements
-- content is loaded in order of increasing loadLevel (loadLevel=0 is loaded first)
-- All data element are by default loadLevel=0
-- All textures are by default loadLevel=1
-- if progressLevel > loadLevel, than this content has been fully loaded
- the load event is called when progressLevel reached "Infinity"
- A progress event is fired whenever the progressLevel increases