Move some gallery helpers into Gallery (rename-only commit to be careful with revision histories)

--HG--
rename : modules/gallery/classes/gallery.php => modules/gallery/classes/Gallery/Gallery.php
rename : modules/gallery/classes/module.php => modules/gallery/classes/Gallery/Module.php
rename : modules/gallery/classes/theme.php => modules/gallery/classes/Gallery/Theme.php
Fix class declarations for some helpers in Gallery.
Add class extensions for some Gallery helpers (and I18n lib).
Merge pull request #223 from shadlaws/gallery_some_helpers_task

Move/rename/extend the other things currently in gallery/classes.