Implemented central storage mechanism for media plugin cookies.

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