Adds a style provider to this, to be used in style construction.
Note that a style provider added by this function only affects the style of the widget to which this belongs. If you want to affect the style of all widgets, use add_provider_for_display.
the priority of the style provider. The lower it is, the earlier it will be used in the style construction. Typically this will be in the range between gtk_style_provider_priority_fallback and gtk_style_provider_priority_user