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New Window Operator (to replace Duplicate Window)

A user doesn't want to necessarily create a new Screen only because she
wants a new window.

This patch allows the user to pick the screen to use for the new Window.
If the screen picked is the active one, it duplicates it (as the old
behaviour in Blender).

Patch with contributions and fixes by Julian Eisel (Severin)

Subscribers: venomgfx

Differential Revision: https://developer.blender.org/D2555