Create GroupDissolvedEvent and react to it

This MR also makes the private group list react to incoming group invitations (once they are implemented).

![device-2016-11-07-155240](/uploads/84ed51cf72bf2c7c597a0fd68b5d868c/device-2016-11-07-155240.png)
![device-2016-11-07-155302](/uploads/446c98d70d674d0bcd79bfb7682dfed5/device-2016-11-07-155302.png)

Closes #736, #737

See merge request !392