Avoid numerous potential PHP warnings when dealing with the pre-r25801 get_core_checksums() return value.

Warnings included current(), filestat(), and md5_file().

see #18201. see #22704.

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git-svn-id: http://core.svn.wordpress.org/trunk@25717 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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


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git-svn-id: http://core.svn.wordpress.org/trunk@25718 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove the no longer used Headerless_Upgrader_Skin class, it's been replaced by Language_Pack_Upgrader_Skin.
See #18200, [25806]

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git-svn-id: http://core.svn.wordpress.org/trunk@25719 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fourteen: first pass at implementing Featured Content settings, props obenland. See #25549.
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git-svn-id: http://core.svn.wordpress.org/trunk@25720 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Avoid printing a possible HTML element. fixes #24813.

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git-svn-id: http://core.svn.wordpress.org/trunk@25721 1a063a9b-81f0-0310-95a4-ce76da25c4cd