LibGUI: Implement content_margins for GroupBox

Author: https://github.com/frhun
Commit: 4955769ab8
Pull-request: https://github.com/SerenityOS/serenity/pull/10246
Issue: https://github.com/SerenityOS/serenity/issues/5887
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibGUI/GroupBox.h

@@ -17,6 +17,7 @@ public:
 
     String title() const { return m_title; }
     void set_title(const StringView&);
+    virtual Margins content_margins() const override;
 
 protected:
     explicit GroupBox(const StringView& title = {});