gnu: xfce4-panel: Support panel plugins from other packages.

* gnu/packages/patches/xfce4-panel-plugins.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/xfce.scm (xfce4-panel): Add the patch, and a
  native search path specification for X_XFCE4_LIB_DIRS.