Relative path support for linked files
Separate "Advanced" prefpane into "General" and "Files and Folders"
Move zotero.org version header pref to Advanced

Translator updates and broken translator reporting both make automatic
requests to zotero.org, but the version pref is just adding a header
with the Zotero version on existing requests.
Relative path support changes

- Changed placeholder to just "attachments:" for consistency with "storage:"
- Reworked dialog text
- Use a fancier (and undocumented) filefield XUL element for path
- A few small code tweaks
More relative path changes

- If attachment exists at same relative path in new base directory,
  leave it alone so that it continues to work.
- If attachment doesn't exist in new base directory, revert it to an
  absolute path.
- If new base directory is an ancestor or descendant of the previous
  base directory, adjust relative paths below the new directory so that
  they keep working.
- More dialog changes
- Select current base directory in file picker when changing directory
- Always use .persistentDescriptor instead of initWithPath(), though it
  probably doesn't matter, and wrap in try/catch in case the old setting
  is broken in some way.
- New function Zotero.File.directoryContains(dir, file), since
  nsIFile.contains() isn't recursive
- Don't use a private Zotero.Item property from outside to force path changes.
Code style/consistency updates for relative path changes
Move relative paths pref code into its own object
Merge branch 'relative_paths' into 3.1

Closes #51