Close windows instead of destroying them

spec/window-helpers.js

@@ -0,0 +1,13 @@
+exports.closeWindow = (window) => {
+  if (window == null || window.isDestroyed()) {
+    return Promise.resolve()
+  } else {
+    return new Promise((resolve, reject) => {
+      window.once('closed', () => {
+        resolve()
+      })
+      window.setClosable(true)
+      window.close()
+    })
+  }
+}