patch 9.0.0597: cannot close a tab page with the middle mouse button

Problem:    Cannot close a tab page with the middle mouse button.
Solution:   Support closing a tab page with the middle mouse button, like many
            other programs. (closes #10746)
diff --git a/src/gui_gtk_x11.c b/src/gui_gtk_x11.c
index 3cdf222..945cb09 100644
--- a/src/gui_gtk_x11.c
+++ b/src/gui_gtk_x11.c
@@ -3345,6 +3345,12 @@
 		send_tabline_event(x < 50 ? -1 : 0);
 	    }
 	}
+	else if (bevent->button == 2)
+	{
+	    if (clicked_page != 0)
+		// Middle mouse click on tabpage label closes that tab.
+		send_tabline_menu_event(clicked_page, TABLINE_MENU_CLOSE);
+	}
     }
 
     // We didn't handle the event.