Port to GtkSource* API

There are still bugs but this gets it started

data/org.x.editor.gschema.xml.in

@@ -155,6 +155,12 @@
       <description>Whether xed should enable syntax highlighting.</description>
     </key>
 
+    <key name="ensure-trailing-newline" type="b">
+      <default>true</default>
+      <summary>Ensure Trailing Newline</summary>
+      <description>Whether gedit will ensure that documents always end with a trailing newline.</description>
+    </key>
+
   </schema>
 
   <schema gettext-domain="@GETTEXT_PACKAGE@" id="org.x.editor.preferences.ui" path="/org/x/editor/preferences/ui/">