The quicklauncher plugin can not be added to the panel. The following error message is written to ~/.xsession-errors:

(process:26496): xfce4-panel-wrapper-CRITICAL **: Wrapper quicklauncher-1: Failed to open plugin module "/usr/lib/xfce4/panel-plugins/libquicklauncher.so": /usr/lib/libxfcegui4.so.4: undefined symbol: xfce_setenv.

xfce_setenv is mentioned here to be remove for xfce 4.12: http://wiki.xfce.org/releng/4.12/roadmap/libxfce4util

xfce_setenv is called in libxfcegui4:
libxfcegui4/xfce-exec.c:    xfce_setenv ("DISPLAY", dpyname, TRUE);


Distribution: Manjaro xfce-minimal
xfce4-quicklauncher-plugin: 1.9.4-7
xfce4-panel: 4.11.0-1