Save selected color_mode using glib settings

de.frajul.music-reader.gschema.xml

@@ -6,5 +6,9 @@
       <default>false</default>
       <summary>Whether the application is in fullscreen mode</summary>
     </key>
+    <key name="color-mode" type="s">
+      <default>'Normal'</default>
+      <summary>The color mode used to render the pdf pages</summary>
+    </key>
   </schema>
 </schemalist>