Refactor settings & session

[tjguk]

The code to handle settings and sessions is scattered around the codebase a little, exacerbated by the need to add baseline packages for the venv PR #1072. There are also other more or less directly-related issues which might be gathered into an overall PR:

#976 -- override UI language
#693 -- start-up switches
#960 -- amnesia mode
#625 -- multiple workspace directories
#1185 -- bring baseline packages into settings

[tjguk]

The main work has now been done via PR #1200 -- the other issues identified above can be addressed separately.