Language Packs: Many many fixes such as:
- Add a "Update Translations" stand-alone button to the updates page
- Shift Language feedback to before update process completion action links & limit the verbosity of output (name + success/errors)
- Simplify/combine the language update descriptive string to only include a plugin/theme name
- Properly handle cache clearing after language updates to prevent langs being repeditively updated
- Display a "All items up to date" string when there's nothing to do
- Reduce the 'Connection Information' from a <h2> to a <h3> to remove duplicate h2's and screen icons from update screens
- Fix the Direct filesystem method not being used for Language updates because WP_LANG_DIR doesn't exist (check it's parent for writable instead)
See #18200, #22704



git-svn-id: https://develop.svn.wordpress.org/trunk@25806 602fd350-edb4-49c9-b593-d223f7449a82
Remove the no longer used Headerless_Upgrader_Skin class, it's been replaced by Language_Pack_Upgrader_Skin.
See #18200, [25806]


git-svn-id: https://develop.svn.wordpress.org/trunk@25807 602fd350-edb4-49c9-b593-d223f7449a82