Add Ctrl-D shortcut to detach composer

https://bugzilla.gnome.org/show_bug.cgi?id=741574

ui/composer.glade

@@ -195,6 +195,7 @@
         <property name="tooltip" translatable="yes">Detach</property>
         <property name="icon_name">detach-symbolic</property>
       </object>
+      <accelerator key="d" modifiers="GDK_CONTROL_MASK"/>
     </child>
     <child>
       <object class="GtkAction" id="send">

ui/composer_accelerators.ui

@@ -36,5 +36,6 @@
     <accelerator action="font_serif" />
     <accelerator action="font_monospace" />
     
+    <accelerator action="detach" />
     <accelerator action="close" />
 </ui>