Item2515: Add support for underline and line-through text decorations

1. TinyMCEPlugin.
   • It generates <span style="text-decoration: ....">foo</span> markup for underlined text.
   • It can apply <del> tags to selected text where line-through formatting is desired.
   • Need to add underline and line-through formatting buttons to the toolbar.
2. WyswiygPlugin? .
   1. HTML2TML? : It must convert TinyMCE? 's output to and from the nominated TML markup
      • Consensus is to go with HTML 3.2 style <u> and <del> tags in the topic text.
      • The <del> tags emitted by HTML2TML? should preserve the incoming class attribute in the output for the benefit of Foswiki::Merge
        • Reasons for <del> instead of <s> or <strike>:
          • <s> is undefined and <strike> is deprecated in XHTML1.0-strict, whereas <del> can pass through Foswiki::Render unchanged.
          • <del> adds semantic meaning to the text it is marking up.
            
            This assumes you never want line-through formatting just for its visual appearance (ie. without implying that the text is somehow not current).
   2. TML2HTML? :
      1. It must convert <u> tags to <span style="text-decoration: ....">foo</span> markup for TinyMCE? .
      2. It must also preserve a class attribute on the <del> tag for the benefit of Foswiki::Merge.
3. Foswiki::Render. It has to convert our nominated TML markup into valid XHTML (strict ?) markup.
   • <u> tags should be converted into XHTML1.0-strict valid styled spans:
     • <span style="text-decoration: underline">foo</span>
     • NB: <u> is undefined in XHTML1.0-strict, but we can use it in our topics as long as Foswiki::Render converts it.
   • <del> tags can be left alone.
4. Foswiki::Merge . It should add the class foswikiMerge to any <ins> and <del> tags that it inserts into a topic, so that they can be distinguished from user applied <del> markup, for the benefit of any conflict resolution utility that might exist in the future.