Added LLPluginCookieStore, which manages the central list of cookies.
New Mac and Windows versions of llqtwebkit, built from the tip of the cookie-api branch on http://bitbucket.org/lindenlab/llqtwebkit/ (currently revision f35a5eab8c2f).
Added "set_cookies" and "cookie_set" messages to the media_browser message class in the plugin API, and made the webkit plugin use them appropriately.
Added methods to LLViewerMedia to read/write the cookie file and add/remove individual cookies.
Added hooks to read/write the cookie file (plugin_cookies.txt) in the same places as the location history (idle_startup() in llstartup.cpp and LLAppViewer::cleanup(), respectively).
Reviewed by Richard at http://codereview.lindenlab.com/1006003
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