Improve encoding dialog

Closes #28 (closed).

This MR fixes various bugs and confusing behaviors of the encoding dialog, and addresses #28 (closed), by always ending up proposing something to the user, as most other text editors do.

To achieve this, I used g_utf8_make_valid(), which is only available since GLib 2.52 (we require GLib 2.42). So I simply copied the source code of current version 2.62 in mousepad/mousepad-util.[ch] with a code switch.

I'm not quite sure if this is legally sufficient in form (author's quotation, license, etc.), so it would be nice if someone who knows these issues better than I do could take a look. Thanks!

Here is a summary of the changes:

• Always show something to the user:
  • if convert() succeeded, show the list of valid encodings;
  • else if convert() partially succeeded (didn't return NULL but not UTF-8 valid), show the list of partially valid encodings (made valid by g_utf8_make_valid());
  • else (unlikely) show the output of g_utf8_make_valid().
• Hide unused widgets:
  • don't show the user a button or a list item that only leads to an error;
  • always show the default UTF-8, since it never fails (but possibly hide the radio button if there are no others).
  • treat the system encoding as a special case:
    • if it is the same as default UTF-8, hide it;
    • else show it, even if it is an unsupported charset (the user should be informed what is his system charset, and if it is supported or not).
• Update the dialog header consequently.