Moved shutdown code from close() to onShutdown().

Note:
There was a lengthy discussion where the shutdown code should go,
according to the information we had initially, close() the right place,
but it turned out that it's not:
https://developer.thunderbird.net/add-ons/mailextensions/experiments#managing-your-experiments-lifecycle

addon/experiments.js

@@ -110,8 +110,43 @@ var pEp4Tb = class extends ExtensionCommon.ExtensionAPI {
   }
 
   onShutdown(isAppShutdown) {
-    if (isAppShutdown) return;
     console.info("pEp4Tb: onShutdown() called");
+
+    ExtensionSupport.unregisterWindowListener(EXTENSION_NAME);
+
+    for (let win of Services.wm.getEnumerator("mail:3pane")) {
+      unpaint(win);
+    }
+    for (let win of Services.wm.getEnumerator("msgcompose")) {
+      unpaint(win);
+    }
+
+    // Unload all modules.
+    Cu.unload("chrome://pEp4Tb/content/modules/handshake.js");
+    Cu.unload("chrome://pEp4Tb/content/modules/pEp.js");
+    Cu.unload("chrome://pEp4Tb/content/modules/pEpAdapter.js");
+    Cu.unload("chrome://pEp4Tb/content/modules/pEpServer.js");
+    Cu.unload("chrome://pEp4Tb/content/modules/utils.js");
+    Cu.unload("chrome://pEp4Tb/content/modules/xhrQueue.js");
+
+    Cu.unload("chrome://pEp4Tb/content/constants.js");
+    Cu.unload("chrome://pEp4Tb/content/detectAdapter.js");
+    Cu.unload("chrome://pEp4Tb/content/pEpController.js");
+    Cu.unload("chrome://pEp4Tb/content/pEpFactory.js");
+    Cu.unload("chrome://pEp4Tb/content/pEpForThunderbird.js");
+    Cu.unload("chrome://pEp4Tb/content/pEpLogger.js");
+    Cu.unload("chrome://pEp4Tb/content/pEpMimeDecrypt.js");
+    Cu.unload("chrome://pEp4Tb/content/pEpMimeEncrypt.js");
+    Cu.unload("chrome://pEp4Tb/content/TbContacts.js");
+    Cu.unload("chrome://pEp4Tb/content/TbHelper.js");
+    // Sync.
+    Cu.unload("chrome://pEp4Tb/content/sendMessage.js");
+    Cu.unload("chrome://pEp4Tb/content/syncWizard.js");
+    Cu.unload("chrome://pEp4Tb/content/TbSync.js");
+    Cu.unload("chrome://pEp4Tb/content/modules/sync.js");
+
+    if (isAppShutdown) return;
+
     this.chromeHandle.destruct();
     this.chromeHandle = null;
     // Looks like we got uninstalled. Maybe a new version will be installed now.
@@ -128,7 +163,6 @@ var pEp4Tb = class extends ExtensionCommon.ExtensionAPI {
 
   getAPI(context) {
     let { extension } = context;
-    context.callOnClose(this);
     return {
       pEp4Tb: {
         addWindowListener() {
@@ -187,42 +221,6 @@ var pEp4Tb = class extends ExtensionCommon.ExtensionAPI {
       },
     };
   }
-
-  close() {
-    console.info("pEp4Tb: close() called");
-    ExtensionSupport.unregisterWindowListener(EXTENSION_NAME);
-
-    for (let win of Services.wm.getEnumerator("mail:3pane")) {
-      unpaint(win);
-    }
-    for (let win of Services.wm.getEnumerator("msgcompose")) {
-      unpaint(win);
-    }
-
-    // Unload all modules.
-    Cu.unload("chrome://pEp4Tb/content/modules/handshake.js");
-    Cu.unload("chrome://pEp4Tb/content/modules/pEp.js");
-    Cu.unload("chrome://pEp4Tb/content/modules/pEpAdapter.js");
-    Cu.unload("chrome://pEp4Tb/content/modules/pEpServer.js");
-    Cu.unload("chrome://pEp4Tb/content/modules/utils.js");
-    Cu.unload("chrome://pEp4Tb/content/modules/xhrQueue.js");
-
-    Cu.unload("chrome://pEp4Tb/content/constants.js");
-    Cu.unload("chrome://pEp4Tb/content/detectAdapter.js");
-    Cu.unload("chrome://pEp4Tb/content/pEpController.js");
-    Cu.unload("chrome://pEp4Tb/content/pEpFactory.js");
-    Cu.unload("chrome://pEp4Tb/content/pEpForThunderbird.js");
-    Cu.unload("chrome://pEp4Tb/content/pEpLogger.js");
-    Cu.unload("chrome://pEp4Tb/content/pEpMimeDecrypt.js");
-    Cu.unload("chrome://pEp4Tb/content/pEpMimeEncrypt.js");
-    Cu.unload("chrome://pEp4Tb/content/TbContacts.js");
-    Cu.unload("chrome://pEp4Tb/content/TbHelper.js");
-    // Sync.
-    Cu.unload("chrome://pEp4Tb/content/sendMessage.js");
-    Cu.unload("chrome://pEp4Tb/content/syncWizard.js");
-    Cu.unload("chrome://pEp4Tb/content/TbSync.js");
-    Cu.unload("chrome://pEp4Tb/content/modules/sync.js");
-  }
 };
 
 function paint(win) {