SMS templates implemented (#439)

* Moved SMS extension to extensions tab.
* Implemented SMS template encoding for Radioddity GD73.
* Implemented SMS templates for TyT/Retevis devices. Addresses 413.
* Implemented message templates for radioddity devices. Addresses #413.
* Fixed encoding of SMS templates for AnyTone devices. Addresses #413.

doc/manual/codeplug/extensions.xml

@@ -25,5 +25,5 @@
 <xi:include href="radioddity/extensions.xml"/>
 <xi:include href="tyt/extensions.xml"/>
 <xi:include href="anytone/extensions.xml"/>
-
+<xi:include href="smsextension.xml"/>
 </chapter>

doc/manual/codeplug/smsextension.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<section xml:id="extMessages"
+         xmlns="http://docbook.org/ns/docbook"
+         xmlns:xlink="http://www.w3.org/1999/xlink"
+         xmlns:xi="http://www.w3.org/2001/XInclude">
+
+<info>
+  <title></title>
+</info>
+
+<para>
+</para>
+
+</section>