Update xfce4-panel to 4.13.0:

4.13.0
======
- Port to Gtk3:
    The port is the result of the hard work Nick, Peter, Andrzejr, Simon and
    many others that helped with testing, patches and translations.
- Features:
  - Support for the "primary" monitor option in RandR
  - Gtk2 plugins are still supported and work almost completely fine
    (see regressions below)
- Deprecations:
  - xfce_panel_image is deprecated, instead plugins shall use GtkImage and
    xfce_panel_plugin_get_icon_size which introduces defined icon size-steps
    to avoid fuzzy icons
- Known regressions:
  - The panel is not visually focused by default (fixed in Xfwm4>=4.12.4)
  - Moving panel plugins via drag and drop on the panel directly via the plugins
    context menu action "Move" is currently broken (works fine in preferences dialog)
  - Small regression with intelligent hiding (when opening a menu, the panel
    hides and comes back immediately while it should just remain visible)
  - Enter/leave opacity does not work for Gtk2 plugins
  - Not all panel settings may be kept as the configuration is not 100% backward
    compatible (e.g. we're using GdkRGBA instead of GdkColor for the backgorund)
- Known todos:
  - Fix all bugs/regressions
  - Remove more/all Gtk+ deprecations
    (lots of warnings when compiling currently)
- Lots of translations updates

4.12.1
======
- Long overdue maintenance release.
- Translation updates.
- Clock: fixed compiler warnings.
- Preferences: support for xfpanel-switch (if installed).
- DirectoryMenu: fixed bugs #10331 and #11742
- Tasklist: fixed background color issue.
- ApplicationsMenu: fixed uninitialized variable.
- make distcheck fix/workaround.

(youri)