Add skinning package support back along with skin selection

indra/llvfs/lldir.cpp

indra/newview/CMakeLists.txt

indra/newview/alunzip.cpp

indra/newview/alunzip.h

indra/newview/llfilepicker.cpp

indra/newview/llfilepicker.h

indra/newview/llfloaterpreference.cpp

indra/newview/llfloaterpreference.h

indra/newview/skins/default/manifest.json

indra/newview/skins/default/xui/en/floater_preferences.xml

indra/newview/skins/default/xui/en/notifications.xml

.../newview/skins/default/xui/en/panel_preferences_skins.xml

indra/newview/skins/default/xui/en/strings.xml

indra/newview/viewer_manifest.py