Port Attacher codde to the extension framework

AttachableObjects are desired in multiple occasions, and the current AttachableObject is not flexible enough to handle all cases. Hence the code is portet to an extension, which gives the needed flexibility.

src/App/DocumentObject.cpp

@@ -82,10 +82,9 @@ DocumentObjectExecReturn *DocumentObject::execute(void)
 {
     //call all extensions
     auto vector = getExtensionsDerivedFromType<App::DocumentObjectExtension>();
-    for(auto ext : vector) {
-        if(ext->extensionMustExecute()) 
-            ext->extensionExecute();
-    }
+    for(auto ext : vector)
+        ext->extensionExecute();
+
     return StdReturn;
 }