selection_edge: map the window once

[N-R-K]

This works okay in my setup but some more testing across different set up would be nice.

[N-R-K]

It looked a bit weird to me that it was mapping a mapped window again and again.

But according to the manpage:

This function has no effect if the window is already mapped.

I guess this is fine to keep it since there's no downside.