stylecontext: Add GTK_STYLE_CLASS_OVERSHOOT

This class is meant to render the hint on scrolledwindow corners
when scrolling past-limits.

https://bugzilla.gnome.org/show_bug.cgi?id=731297

docs/reference/gtk/gtk3-sections.txt

@@ -6001,6 +6001,7 @@ GTK_STYLE_CLASS_MESSAGE_DIALOG
 GTK_STYLE_CLASS_NEEDS_ATTENTION
 GTK_STYLE_CLASS_NOTEBOOK
 GTK_STYLE_CLASS_OSD
+GTK_STYLE_CLASS_OVERSHOOT
 GTK_STYLE_CLASS_PANE_SEPARATOR
 GTK_STYLE_CLASS_POPUP
 GTK_STYLE_CLASS_PRIMARY_TOOLBAR

gtk/gtkstylecontext.h

@@ -863,6 +863,16 @@ struct _GtkStyleContextClass
  */
 #define GTK_STYLE_CLASS_READ_ONLY "read-only"
 
+/*
+ * GTK_STYLE_CLASS_OVERSHOOT:
+ * A CSS class that is added on the visual hints that happen
+ * when scrolling is attempted past the limits of a scrollable
+ * area.
+ *
+ * Since: 3.14
+ */
+#define GTK_STYLE_CLASS_OVERSHOOT "overshoot"
+
 /**
  * GTK_STYLE_REGION_ROW:
  *