Focus Pext when launching second instance on Windows

[TheLastProject]

This already works on Linux and macOS, but Windows currently doesn't support the way this is done. Will need fixing at some point.

The problem is specifically around

Pext/pext/__main__.py

Lines 2636 to 2640 in a1b5a01

	if platform.system() == 'Windows':
	print("Pext is already running and foregrounding the running instance is currently not supported "
	"on Windows. Doing nothing...")
	else:
	os.kill(int(open(pidfile, 'r').read()), signal.SIGUSR1)

and

Pext/pext/__main__.py

Lines 3034 to 3035 in a1b5a01

	if not platform.system() == 'Windows':
	signal.signal(signal.SIGUSR1, signal_handler.handle)

, because Windows doesn't support signals like this.