Theme apertis sdk

The current SDK theme is a mix of clashing colours (highlighting in green,
deep purple background). Apply "xfce-4.6" theme to chrome, add custom
Apertis wallpaper.

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

overlays/arch-sdk/etc/skel/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-desktop.xml

+<?xml version="1.0" encoding="UTF-8"?>
+
+<channel name="xfce4-desktop" version="1.0">
+  <property name="backdrop" type="empty">
+    <property name="screen0" type="empty">
+      <property name="monitor0" type="empty">
+        <property name="image-path" type="empty"/>
+        <property name="image-show" type="empty"/>
+        <property name="color1" type="empty"/>
+        <property name="workspace0" type="empty">
+          <property name="color-style" type="int" value="0"/>
+          <property name="color1" type="array">
+            <value type="uint" value="58136"/>
+            <value type="uint" value="58136"/>
+            <value type="uint" value="58136"/>
+            <value type="uint" value="65535"/>
+          </property>
+          <property name="image-style" type="int" value="5"/>
+          <property name="last-image" type="string" value="/usr/share/sac-sdk/wallpapers/apertis-desktop.png"/>
+        </property>
+      </property>
+    </property>
+  </property>
+</channel>
+

overlays/arch-sdk/etc/skel/.config/xfce4/xfconf/xfce-perchannel-xml/xsettings.xml

+<?xml version="1.0" encoding="UTF-8"?>
+
+<channel name="xsettings" version="1.0">
+  <property name="Net" type="empty">
+    <property name="ThemeName" type="string" value="Xfce-4.6"/>
+  </property>
+</channel>