Properly handle closing of displayed windows

[karlmsmith]

When the close button in the window frame of displayed windows is selected, the window should close and this information sent back to the Ferret engine to let it know the window was cancelled. Current released behavior is to just minimize the window, but this complicates closing a window when PyFerret crashes and leaves a window behind.

Already updated (unreleased) to allow the window to close, but still need to get the message back to the Ferret engine.

Also need to add closing of the pipes to displayed windows to the method registered with atexit so windows are not left behind on crashes.