create hide_edge_borders option

include/con.h

@@ -221,6 +221,12 @@ Con *con_descend_direction(Con *con, direction_t direction);
  */
 Rect con_border_style_rect(Con *con);
 
+/**
+ * Returns adjacent borders of the window. We need this if hide_edge_borders is
+ * enabled.
+ */
+adjacent_t con_adjacent_borders(Con *con);
+
 /**
  * Use this function to get a container’s border style. This is important
  * because when inside a stack, the border style is always BS_NORMAL.