Add a default XFCE4 panel configuration

We currently don't provide a default panel configuration. As a result the
user is getting multiple errors at boot, reading "No running instance of
xfce4-panel was found". Depending on the how the user navigates these
messages, the user may find themselves presented with the message
"Modifying the panel is not allowed" on every subsequent boot.

Provide default panel configuration to improve the user experience and to
avoid them being presented with errors.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

overlays/sdk-xfce-theme/etc/skel/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-panel.xml

+<?xml version="1.0" encoding="UTF-8"?>
+
+<channel name="xfce4-panel" version="1.0">
+  <property name="configver" type="int" value="2"/>
+  <property name="panels" type="array">
+    <value type="int" value="1"/>
+    <property name="panel-1" type="empty">
+      <property name="position" type="string" value="p=6;x=0;y=0"/>
+      <property name="length" type="uint" value="100"/>
+      <property name="position-locked" type="bool" value="true"/>
+      <property name="size" type="uint" value="30"/>
+      <property name="plugin-ids" type="array">
+        <value type="int" value="1"/>
+        <value type="int" value="3"/>
+        <value type="int" value="15"/>
+        <value type="int" value="4"/>
+        <value type="int" value="5"/>
+        <value type="int" value="6"/>
+      </property>
+    </property>
+  </property>
+  <property name="plugins" type="empty">
+    <property name="plugin-1" type="string" value="applicationsmenu"/>
+    <property name="plugin-3" type="string" value="tasklist">
+      <property name="show-handle" type="bool" value="false"/>
+    </property>
+    <property name="plugin-15" type="string" value="separator">
+      <property name="expand" type="bool" value="true"/>
+      <property name="style" type="uint" value="0"/>
+    </property>
+    <property name="plugin-4" type="string" value="pager"/>
+    <property name="plugin-5" type="string" value="clock"/>
+    <property name="plugin-6" type="string" value="systray">
+      <property name="show-frame" type="bool" value="false"/>
+    </property>
+  </property>
+</channel>