[HighGUI] On Windows: Support Ctrl+C to copy image to clipboard.

Added support for copying the image to the clipboard.
I tried various versions of inserting the Ctrl+C handler into the code. 
This is what worked. 
As I said, I don't completely understand the the proc flow as it is implemented, but some things are not passed-on and others skip directly further down.

This is how I got it to work with as few trade-offs as possible.