win-capture: Use window for keepalive check

To check to make sure game capture is still active in the capture
program, it currently uses a named event, and then it checks to see if
that named event exists.  However with UWP programs, you can't open a
named event outside of the UWP process.  FindWindow on the other hand
does work, so instead of checking to see if a named kernel object
exists, create a window and check to see if that window exists.