fix: apparently setters shadow getting

I thought you could:
  method() {}
  set method (v) { /* ... /* }

but you can't. You have to:
  get method() { return (function () {}).bind(this) }
  set method (v) { /* ... /* }

I don't know why a setter can't just only shadow setting, but that's
how they designed the language. ¯\_(ツ)_/¯

src/backend/src/Extension.js

@@ -219,10 +219,12 @@ class Extension extends AdvancedBase {
         });
     }
 
-    preinit (callback) {
-        this.on('preinit', callback);
+    get preinit() {
+        return (function (callback) {
+            this.on('preinit', callback);
+        }).bind(this);
     }
-    set preinit (callback) {
+    set preinit(callback) {
         if ( this.only_one_preinit_fn === null ) {
             this.on('preinit', (...a) => {
                 this.only_one_preinit_fn(...a);
@@ -233,9 +235,11 @@ class Extension extends AdvancedBase {
         }
         this.only_one_preinit_fn = callback;
     }
-    
-    init (callback) {
-        this.on('init', callback);
+
+    get init() {
+        return (function(callback) {
+            this.on('init', callback);
+        }).bind(this);
     }
     set init (callback) {
         if ( this.only_one_init_fn === null ) {
@@ -248,8 +252,6 @@ class Extension extends AdvancedBase {
         }
         this.only_one_init_fn = callback;
     }
-    
-    //
 
     /**
      * This method will create the "default service" for an extension.