Adds a new offset marker on this at the position specified by value.
When the bar value is in the interval topped by value (or between value and [[email protected]: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.
| this |
a `GtkLevelBar` |
| name |
the name of the new offset |
| value |
the value for the new offset |