Event cleanup

This is a larger code cleanup and gets rid of lively.EventExperiments.

With evt.getTargetMorph() we get a fast and central mechanism to lookup morphs that are event targets. This allows to put "meta event handling" (halos, "that", drag dispatch, etc.) in the mouse down/up/move handlers of the World which considerably frees the default mouse(down/up)entry methods of Morph.

Caveat, though: This mechanism uses the DOM (which makes it fast) and does not consider specific Morphic handling. E.g. when "grabbing" a morph and trying to find a drop target for it this mechanism can not be used because the grabbed morph will "block" other morphs. For this we still need #morphsContainingPoint.