Added static member functions to LLViewerMedia: clearAllCookies(), clearAllCaches(), setCookiesEnabled(), and setProxyConfig().  These iterate through all loaded instances and send appropriate messages.

Also added a hack to clearAllCookies() to delete cookie files for all accounts, which should serve until we centralize cookie storage.

Made LLViewerMedia set the proxy config properly when creating new instances.  Callum is adding code to do something similar with the cookies enabled flag in a separate commit.

Removed get_web_media from llfloaterpreference.cpp and reworked everything that used it to use the new LLViewerMedia functions instead.

Reviewed by Callum at http://codereview.lindenlab.com/838003.