../GtkFrame is a widget to draw frames around widgets, or to group them.
LABEL_PAD is a 1 pixel padding added to the size of the label. LABEL_SIDE_PAD is a padding on both sides of the frame, where the label cannot be moved to. The label itself can be aligned anywhere on the frame.
../GtkFrame draws a SHADOW or SHADOW_GAP, depending on whether it has a label or not. The shadow can be changed by applications and defaults to ETCHED_IN.