Work on mail client

- Registry value /org/luwrain/pim/mail/group-outgoing-uri added to mark the group for composed messages
- MessageApp gets first code for saving composed messages
- MailApp gets first code for opening mail groups
- pim.MailRegistryValues class added for simplifying access to registry params for mail
- Updates for FormArea and MessageArea
- NavigateArea and SimpleArea go to controls package