Edge backend leaks processes

When running a WebView with the Edge backend, two "Desktop App Web Viewer" processes are started. If the WebView window is closed (clicking the "x" or alt+f4), these processes are terminated. However, if `WebView::terminate()` is called instead, these processes are never ended and will outlive the main process.

I messed around a bit with the C++ implementation of the Edge backend, and I could only get one of the processes to close (via `m_process.Terminate()`).