Adds a mark at value.
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.
If markup is not NULL, text is shown next to the tick mark.
To remove marks from a scale, use Scale.clearMarks.
the value at which the mark is placed, must be between the lower and upper limits of the scales’ adjustment
Text to be shown at the mark, using [Pango markup]PangoMarkupFormat, or NULL