[GREENFOOT-825] Window invisible on startup of coordinates out of screen

It seems that the main scenario window can be out of screen. It should automatically be moved into the visible screen area on scenario open.

This also affects windows that were minimised in MS Windows, since they are assigned a nominal window position of -32000 / -32000.

Reported in Greenroom (26 Mar 2019: [Greenfoot Scenario starts invisible|https://greenroom.greenfoot.org/forums/1/topics/630]).

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