Fix for bug NB#74675. Now we limit the retrieval size for texts shown,

Fix for bug NB#74675. Now we limit the retrieval size for texts shown,
and make the load much more efficient.
* src/modest-text-utils.[ch]:
        * Now convert_to_html_body method accepts two new parameters,
          one to enable disable linkify and other for processing char
          buffers with a size limit.
        * Now we provide the methods modest_text_utils_linkify_begin
          and modest_text_utils_linkify_end, to cache the regex patterns
          in processing of a big set of strings.
* src/widgets/modest-gtkhtml-mime-part-view.c:
        * Now we use the new ModestStreamTextToHtml decorator for
          gtkhtml stream instead of a big hack we did before to
          convert the text to html and send it to the gtkhtml widget.
        * Now we set the maximum size of text with hyperlinkify (64KB)
          and a maximum size of the text document to show (640KB).
* src/modest-stream-text-to-html.[ch], src/Makefile.am:
        * New stream implementation that proxies a write stream. With
          this all text written to this stream is converted to html and
          sent to the decorated stream.

pmo-trunk-r3607