input panel: Add drawing of layout

input panel: Add drawing of layout

The drawing makes clear in a few seconds:
 - what columnT and columnB stand for
 - what additional containers and panels are created
 - to which of those the elements of column1, column2... go to

When you're in the JS debugger and lost overview of where in this
element hierarchy you are, you can quickly check xtype + layout. Then
consulting this drawing solves the mistery.

Signed-off-by: Dominic Jäger <d.jaeger@proxmox.com>