Add experimental new chat history formatting

indra/llui/llstyle.cpp

indra/llui/llstyle.h

indra/llui/lltextbase.cpp

indra/newview/app_settings/settings_alchemy.xml

indra/newview/llchathistory.cpp

indra/newview/llfloaterconversationpreview.cpp

indra/newview/llfloaterimsessiontab.cpp

indra/newview/skins/alchemy/colors.xml

indra/newview/skins/default/colors.xml

...ewview/skins/default/xui/en/menu_im_session_showmodes.xml