xfce4-panel: update to 4.13.6

Change log:

4.13.6
======
- Update default panel layout
- Bugfixes:
  - Retain original GDK_CORE_DEVICE_EVENTS setting for plugins (Bug
  #15044)
  - directorymenu: Fix plugin menu autohide behaviour
  - prefs: Add missing mnemonic accelerators (Bug #15652)
  - prefs: Improve window title (Bug #15653)
  - prefs: Correct capitalization of labels (Bug #15651)
  - arrow-button: Allow connect_after signals on draw
  - tasklist: Draw grouped windows count indicator (Bug #10844)
  - tasklist: Only base bg color of group indicator on Gtk theme
  - tasklist: Avoid draw signal loop
  - tasklist: Don't show actionsmenu in grouped windows (Bug #15622)
  - Add more accelerators to dialog buttons (Bug #15490)
  - AC_CONFIG_MACRO_DIR -> AC_CONFIG_MACRO_DIRS
  - Fix typo in debug output
- Translation updates:
    Albanian, Bulgarian, Catalan, Chinese (China), Chinese (Taiwan),
    Croatian, Czech, Danish, Dutch (Flemish), English (Australia), French,
    German, Italian, Lithuanian, Polish, Portuguese, Portuguese
    (Brazilian), Serbian, Spanish, Turkish

(gutteridge)