New preferences dialog

[Seán de Búrca <leftmostcat gmail com>]

    As I write this, I am reviewing changes I've made to commit a
rewritten preferences dialog to the GOBJECT_WORK branch, as I discussed
with Nacho. Before I do so, though, I have a few notes.

    First, I've done a bunch of renaming and rewording. Everything
builds fine for me and works as well but I can't be certain that it does
for everyone. Please try to break it. A few settings have been renamed
and as such you will lose your current settings when you update. The
following have changed:

sweep_compile_file -> delete_compiled
autosave_timeout -> autosave_interval
autosave_with_suffix -> append_suffix
use_dot_char -> visible_whitespace
own_fonts -> use_custom_font
text_font -> editor_font
instant_spell_checking -> spellcheck
authors_name -> name
authors_email -> email
plural -> plurals

    I have also removed plural_note as I don't see any use to this.
Please let me know if this is in error and I'll correct it right away. A
few other settings which are not currently used have also been renamed.
I personally feel that the above renames are meaningful but I am open to
discussion. If any of the changes I've made strike you as wrong, let me
know and we can try to work it out so that everyone is happy. I have a
few ideas about how the current system can be changed and improved but
for the time being I'm most interested in general feedback about the
dialog itself and the code changes I've made. Thanks for your work and
feedback!

    I've made screenshots of the new dialog available for those who
would prefer to see it before they update:

http://www.leftmostcat.net/gtranslator-files.png
http://www.leftmostcat.net/gtranslator-editor.png
http://www.leftmostcat.net/gtranslator-poheader.png


Seán de Búrca