feat: add schema sidebar position (#326)

Author: alainm23

data/settings.gschema.xml

@@ -15,6 +15,11 @@
       <default>830</default>
       <summary>Most recent window width</summary>
       <description>Most recent window width</description>
+    </key>
+    <key name="sidebar-position" type="i">
+      <default>-1</default>
+      <summary>Width of the sidebar</summary>
+      <description>This key stores the width of the sidebar. It is used to restore the sidebar position in future sessions.</description>
     </key>
 	</schema>
 </schemalist>