javagui: eliminated spurious widget construction/destruction

Hi.

javagui triggers (maybe also other backends to the same) widget construction on
setContainer, now given the eager push logic setContainer is/can be called many
times for the same container. With the previous logic the widget would be
destroyed and then (re)constructed each time. With the change this happen only
if the container.widget is different from the previous one. Maybe this
improvement may apply to other backends.

regards.

PS: probably javagui can do keeping the same widget even when the container
change. For the momement that is left unimplemented.



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