Added an additional explanation and code snippet for GtkImageMenuItem.c
Added an additional explanation and code snippet for GtkImageMenuItem.c

https://bugzilla.gnome.org/show_bug.cgi?id=752093

jadahl/gtk,Adamovskiy/gtk,chergert/gtk,Adamovskiy/gtk,davidgumberg/gtk,Adamovskiy/gtk,jadahl/gtk,chergert/gtk,alexlarsson/gtk,davidgumberg/gtk,davidgumberg/gtk,chergert/gtk,chergert/gtk,jadahl/gtk,Adamovskiy/gtk,jigpu/gtk,chergert/gtk,grubersjoe/adwaita,jadahl/gtk,jadahl/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,Adamovskiy/gtk,Adamovskiy/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,davidgumberg/gtk,jadahl/gtk,Adamovskiy/gtk,alexlarsson/gtk,davidgumberg/gtk,jigpu/gtk,alexlarsson/gtk,jigpu/gtk,Adamovskiy/gtk,alexlarsson/gtk,davidgumberg/gtk,jigpu/gtk,davidgumberg/gtk,chergert/gtk,jigpu/gtk,alexlarsson/gtk,jigpu/gtk,jadahl/gtk,jigpu/gtk,grubersjoe/adwaita,davidgumberg/gtk,chergert/gtk,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita