fix(App): Force Franz to use single window

Fixes #29

src/index.js

@@ -18,7 +18,6 @@ import './electron/exception';
 // Keep a global reference of the window object, if you don't, the window will
 // be closed automatically when the JavaScript object is garbage collected.
 let mainWindow;
-const settings = new Settings();
 let willQuitApp = false;
 
 // Ensure that the recipe directory exists
@@ -29,6 +28,21 @@ if (isWindows) {
   app.setAppUserModelId(appId);
 }
 
+// Force single window
+const isSecondInstance = app.makeSingleInstance(() => {
+  if (mainWindow) {
+    if (mainWindow.isMinimized()) mainWindow.restore();
+    mainWindow.focus();
+  }
+});
+
+if (isSecondInstance) {
+  app.quit();
+}
+
+// Initialize Settings
+const settings = new Settings();
+
 const createWindow = async () => {
   // Remember window size
   const mainWindowState = windowStateKeeper({