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.h

@@ -52,6 +52,8 @@ public:
     void update_diff();
     Vector<Diff::Hunk> const& hunks() const { return m_hunks; }
 
+    Function<void()> on_change;
+
 private:
     EditorWrapper();