Private Group List UI

This MR implements the UI for the list of private groups.

It reacts to three types of events to refresh the displayed data:
* new group message received
* private group added
* private group removed

Missing from final implementation:
* entering groups
* adding new groups
* reacting to a future group dissolved event
* actually removing a dissolved group

![device-2016-09-29-180741](/uploads/666f04e8c9e2c81bdfe7f5648c14e71d/device-2016-09-29-180741.png)
![device-2016-10-03-141200](/uploads/5d0a54ea5a31d64404591c03ccf1e3b6/device-2016-10-03-141200.png)
![groups](/uploads/3eba757e21837739a129aab15100c06a/groups.gif)

Closes #660

See merge request !335