HackStudio: Fix editor not marking file with unsaved changes as dirty

The editor's on_change callback was being overwritten in
HackStudioWidget.cpp in order to call update_gml_preview on every
change. This stopped the original callback from being called and marking
files as dirty when changed.

Now we call update_gml_preview in a new callback within the editor
wrapper, which is then called within the original on_change callback
in the editor.

Userland/DevTools/HackStudio/EditorWrapper.cpp

@@ -43,6 +43,8 @@ EditorWrapper::EditorWrapper()
     };
 
     m_editor->on_change = [this] {
+        if (this->on_change)
+            this->on_change();
         bool was_dirty = m_document_dirty;
         m_document_dirty = true;
         if (!was_dirty)