#16 clean close

[Oleksiy Kebkal]

Docking Window doesn't provide any way to cleanly close
the docking window to be cleaned afterwards by the
garbage collector.

For example, if I create dynamically Views, and after
some time close it, the Docking Windows still have a
lot of inner references to the View, so it will be
still marked as used, and it will not be cleaned. So we
get an effect of accumulation of not used by the
program memory.

I've tried to clean the property map of the docking
window, but it doesn't help.
(like view.getWindowProperties().getMap().clear(true))