PlotItem was keeping ViewBox alive (so clearing that cycle)

Fixed one cycle which would keep viewbox and plotitems alive as the plotitem was keeping a reference to instancemethods of the viewbox (which was never removed).

Also, making sure there's no reference to a ButtonItem on close.

And at last, I'm printing things in the exit if objects are still alive (which means things weren't cleared properly) -- feel free to make that a debug only option or skip it altogether if you want (but I think that the other parts are pretty important).