Adds a mark at
A mark is indicated visually by drawing a tick mark next to the scale, and GTK makes it easy for the user to position the scale exactly at the marks value.
markup is not null, text is shown next to the tick mark.
To remove marks from a scale, use clear_marks.
the value at which the mark is placed, must be between the lower and upper limits of the scales’ adjustment
where to draw the mark. For a horizontal scale, TOP and gtk_pos_left are drawn above the scale, anything else below. For a vertical scale, LEFT and gtk_pos_top are drawn to the left of the scale, anything else to the right.
Text to be shown at the mark, using Pango markup, or null