Add checkbox support

src/kmcomposerui.rc

 <?xml version="1.0"?>
 <!DOCTYPE gui SYSTEM "kpartgui.dtd" >
-<gui version="524" name="kmcomposer" translationDomain="kmail">
+<gui version="525" name="kmcomposer" translationDomain="kmail">
  <MenuBar>
   <Menu noMerge="1" name="file" >
    <text>&amp;Message</text>
@@ -132,6 +132,7 @@
   <Action name="format_list_style" />
   <Action name="format_font_family" />
   <Action name="format_font_size" />
+  <Action name="format_list_checkbox" />
   <Separator/>
   <Action name="format_reset" />
   <Separator/>