Clean up locale string editing screen

The little patch included here is part of two issues already (http://drupal.org/node/146033 and http://drupal.org/node/141461), but it is a general cleanup and bugfix patch, so it seemed to be logical to get it in sooner then later to not block/conflict other patches on this.

The included patch does the following:

- properly checks for an existing string to edit
- provides a form with rich metadata for altering for core and contrib modules to provide custom interfaces
- fixes an ordering bug: previously strings were always ordered differently on all string editing pages because the order of existing translations were relevant

I think this is quite simple and ready to go in, but would need reviews anyway.