Split off advanced settings model
It basically works.
Add string support to advanced settings/settings model.
Add some more settings, fix bug with default values in floats being changed to an int.
Add some warnings. Improve iTerWarning behavior when there is a cancel button to not remember anything.
Rip out migration stuff, it's too risky. Also use a different pref for url vs file remote prefs changed warnings. Everyone will get the warning again.
Convert more code to use iTermWarning and iTermSettingsModel.