Increase icon size of status tray in panel

The icons of the status tray plugin were rather small and arranged in two
rows. This was due to the upper panel being quite crowded and increasing
their size causes difficulties on devices with small screen. On the other
hand, more and more touchscreen devices with high resolution show up in
postmarketOS, where it hardly possible to hit the tiny icons.

Being able to hit the "onboard" icon is crucial for touchscreen usage because
this calls or hides the virtual keyboard in situations where it doesn't show
up automatically. But also the network-manager icon the the bluetooth icon are
important icons that should be accessible.

Increasing their icon size is a compromise between facing the growing number
of high-resolution touchscreen devices and preserving usability on older low-
resolution touchscreen devices.

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

@@ -53,7 +53,9 @@
       <property name="style" type="uint" value="0"/>
     </property>
     <property name="plugin-4" type="string" value="pager"/>
-    <property name="plugin-5" type="string" value="systray"/>
+    <property name="plugin-5" type="string" value="systray">
+      <property name="icon-size" type="int" value="30"/>
+    </property>
     <property name="plugin-6" type="string" value="pulseaudio">
       <property name="enable-keyboard-shortcuts" type="bool" value="true"/>
     </property>