Enable nm-applet's application indicator support

Application indicators are supported since xfce4-panel 4.16.0. Update panel's systray configuration accordingly:

- show-frame was removed.

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- size-max was renamed to icon-size. Specifying 0 means "Adjust size automatically".

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- names-visible was renamed to known-items (application indicator) and known-legacy-items (xembed).

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airootfs/root/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-panel.xml

@@ -63,10 +63,9 @@
       <property name="enable-keyboard-shortcuts" type="bool" value="true"/>
     </property>
     <property name="plugin-9" type="string" value="systray">
-      <property name="show-frame" type="bool" value="false"/>
-      <property name="size-max" type="uint" value="24"/>
-      <property name="names-visible" type="array">
-        <value type="string" value="networkmanager applet"/>
+      <property name="icon-size" type="int" value="0"/>
+      <property name="known-items" type="array">
+        <value type="string" value="nm-applet"/>
       </property>
     </property>
     <property name="plugin-10" type="string" value="clock">

airootfs/root/customize_airootfs.sh

@@ -99,6 +99,11 @@ rm -f /usr/share/qt/translations/*
 sed -i '2 i NoDisplay=true' /usr/share/applications/{xfce4-mail-reader,xfce4-web-browser}.desktop
 sed -i "s/^\(Categories=\).*\$/Categories=Utility;/" /usr/share/applications/{geany,*ristretto*,*GHex*}.desktop
 
+# nm-applet with application indicator enabled gives better integration with xfce4-panel's systray
+mkdir -p /root/.config/autostart/ /usr/local/share/applications/
+sed 's/^Exec=nm-applet$/& --indicator/' /etc/xdg/autostart/nm-applet.desktop > /root/.config/autostart/nm-applet.desktop
+sed 's/^Exec=nm-applet$/& --indicator/' /usr/share/applications/nm-applet.desktop > /usr/local/share/applications/nm-applet.desktop
+
 # Remove large/irrelevant firmwares
 rm -rf /usr/lib/firmware/{liquidio,netronome,mellanox,mrvl/prestera,qcom}