Add notification support to Matterhorn container

[BennyLi]

If a new message arrives Matterhorn issues a system notification. This can also be configured via activityNotifyCommand. An example script is in the official repo: notifications-scripts/notify.

Found these workaround:

• [notify-send-http](https://github.com/fgrehm/notify-send-http

Other ways seem to depend on mounting the X11 socket and maybe dbus.

[BennyLi]

Checkout Jessys stuff:

• https://github.com/jessfraz/dotfiles/blob/master/.dockerfunc
• https://github.com/jessfraz/dockerfiles/blob/master/scudcloud/Dockerfile