Sync screen context usage with tryton
From https://codereview.tryton.org/41841002/#msg4 it will be great to sync the screen API on sao with tryton. Some functions that should be added/changed:
- Sao.Screen.new_group should accept a context argument and not a the ids
- Sao.Screen.load is missing
- Sao.screen.context should be a function that returns the group context (like the python property on gtk client).
- Sao.Group.load is also missing