imprv: Add support for the urgency hint on win32

Closes #12341 (closed)

As I normally do C# I'm not sure if the way I have this implemented is to your standards. I'd be happy to amend any changes you would wish for.

This change adds back the flashing taskbar entry in Windows that is not supported anymore since the move to GTK4 (as mentioned in the linked issue). The comment also pointed me in the right direction on where to look and what to implement.