#7 Monitor several devices in one frame

[Christian]

So far, only one device (but with different panes, e.g. histogram and percent) can be monitored in one frame.

Change it to monitor several devices in one frame.

Use case:
- one frame that monitors 4 devices (one pane for each) in one frame, e.g.:
( ("cpu","label-bar"), ("i/o","label-bar"), ("mem","label-bar"), ("swp","label-bar") )