Option to add the installation of an event filter to an existing Win3…

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When integrating Vulkranscenegraph into an existing Win32 application (MFC program) I noticed that there is no event handler. With the flag installEventHandler in the WindowTraits class, the existing event handler in Win32_Window can be used if required.

The default value of the flag is set to False to avoid unwanted side effects (like in vsgQt).

I have adapted the solution from the Openscenegraph project, as you should recognize.