Exit net core backend on electron quit

This change explicitly cleans up the .net core http process when the
hosting electron application quits.  On macos, the child process was
sometimes left running depending on how the application was exited.

GH-346

ElectronNET.Host/main.js

@@ -62,6 +62,11 @@ app.on('ready', () => {
 
 });
 
+app.on('quit', async (event, exitCode) => {
+    await server.close();
+    apiProcess.kill();
+});
+
 function isSplashScreenEnabled() {
     if (manifestJsonFile.hasOwnProperty('splashscreen')) {
         if (manifestJsonFile.splashscreen.hasOwnProperty('imageFile')) {