Remove null callback arg. Fixes #2530955

Fix sub detacher prop and mouseleave context/args Poor due diligence. Detach was trying to detach a handle from the wrong property name (js error). Fixes #2529976 And only the mouseenter callback was executed from the override context or passed additional subscription args. Now both mouseenter and mouseleave report as e.type === 'hover'. The phase is distinguished by e.phase === 'over' or 'out'. And of course the context and args embarrassment is fixed. Fixes #2529977

Add hover synth event. Include in event rollup. Fixes #2529495