mac-capture: Fix creation of invalid window stream on source creation

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.