Support for live unicode character replacement

As requested here (https://bugzilla.redhat.com/show_bug.cgi?id=634789), I looked into adding support for replacing common multi-character strings with their single character unicode equivalent, to minimize character count for microblog services which have a limit (which is pretty much all of them).

Specifically, "..." and "--" can be simplified to "…" and "—".

I hacked together a patch for this, but I admit it is not pretty. It does work for me, but the console doesn't love it, as I get:

found ... trying to replace
/usr/bin/gwibber:73: GtkWarning: Invalid text buffer iterator: either the iterator is uninitialized, or the characters/pixbufs/widgets in the buffer have been modified since the iterator was created.
You must use marks, character numbers, or line numbers to preserve a position across buffer modifications.
You can apply tags and insert marks without invalidating your iterators,
but any mutation that affects 'indexable' buffer contents (contents that can be referred to by character offset)
will invalidate all outstanding iterators
  gtk.main()
/usr/bin/gwibber:73: GtkWarning: _gtk_text_buffer_get_line_log_attrs: assertion `GTK_IS_TEXT_BUFFER (buffer)' failed
  gtk.main()
/usr/bin/gwibber:73: GtkWarning: IA__gtk_text_buffer_remove_tag: assertion `gtk_text_iter_get_buffer (end) == buffer' failed
  gtk.main()

** (gwibber:11855): CRITICAL **: enchant_dict_check: assertion `len' failed
/usr/bin/gwibber:73: GtkWarning: gtk_text_buffer_set_mark: assertion `gtk_text_iter_get_buffer (iter) == buffer' failed
  gtk.main()

I can't seem to find a good example of searching for a string in a textbuf, and replacing it in place whenever found. I did it without using marks or iters, which is why the warnings get thrown. I'm attaching this patch as a starting point, with the hope that someone who understands textbufs better than I do will clean it up a bit so it is no longer such a warning generator.