When the ScreenCaptureKit-based source is generated, the window ID
read from the settings will be empty and interpreted as window ID "0".
This window ID doesn't represent a valid window, but the capture stream
is nevertheless successfully initialized with that value, but will not
generate any output.
Worse, the activated stream should be destroyed when a new stream is
created, but creation will throw an error.
This commit ensures that a valid window ID is used if no good default
value is available, ensuring a working (and destroyable) stream.
Also removes any remaining usage of the old window-utils.
93adc0a88c