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_w32.c b/src/gui_w32.c
index 203b8fa..3ade631 100644
--- a/src/gui_w32.c
+++ b/src/gui_w32.c
@@ -8094,6 +8094,20 @@
 		}
 		break;
 	    }
+	case WM_MBUTTONUP:
+	    {
+		TCHITTESTINFO htinfo;
+
+		htinfo.pt.x = GET_X_LPARAM(lParam);
+		htinfo.pt.y = GET_Y_LPARAM(lParam);
+		idx0 = TabCtrl_HitTest(hwnd, &htinfo);
+		if (idx0 != -1)
+		{
+		    idx0 += 1;
+		    send_tabline_menu_event(idx0, TABLINE_MENU_CLOSE);
+		}
+		break;
+	    }
 	default:
 	    break;
     }