gui2/ttext_box: Restore selection highlight lost in version 1.13.3

It was unexplainedly dropped by commit
67cf95a5af.

changelog

@@ -52,6 +52,7 @@ Version 1.13.5+dev:
      entries (lost in 1.13.3).
    * When using the --wconsole option, the game now prints a prompt in the event
      of a fatal error to avoid closing the console before the error can be seen.
+   * Restored GUI2 textbox selection highlight lost in version 1.13.3.
  * WML Engine:
    * Added ignore_special_locations=yes|no, default no, to [terrain_mask] to
      ignore the special locations given in the mask, leaving all those on the

data/gui/widget/text_box_default.cfg

@@ -24,6 +24,17 @@
 #enddef
 
 #define _GUI_DRAW_BORDER COLOR
+	[rectangle]
+		x = "(text_x_offset + selection_offset)"
+		y = "(text_y_offset)"
+		w = "(selection_width)"
+		h = "(text_font_height)"
+
+		border_thickness = 0
+		fill_color = "21, 53, 80, 255"
+
+	[/rectangle]
+
 	[rectangle]
 		x = 0
 		y = 0