Determines the way `GtkLevelBar` interprets the value properties to draw the level fill area.
Specifically, when the value is gtk_level_bar_mode_continuous, `GtkLevelBar` will draw a single block representing the current value in that area; when the value is gtk_level_bar_mode_discrete, the widget will draw a succession of separate blocks filling the draw area, with the number of blocks being equal to the units separating the integral roundings of [[email protected]:min-value] and [[email protected]:max-value].