Adds a new offset marker on @self at the position specified by @value.
When the bar value is in the interval topped by @value (or between @value
and #GtkLevelBar:max-value in case the offset is the last one on the bar)
a style class named level-@name will be applied
when rendering the level bar fill.
If another offset marker named @name exists, its value will be
replaced by @value.
Adds a new offset marker on @self at the position specified by @value. When the bar value is in the interval topped by @value (or between @value and #GtkLevelBar:max-value in case the offset is the last one on the bar) a style class named level-@name will be applied when rendering the level bar fill. If another offset marker named @name exists, its value will be replaced by @value.