input panel: support convenience top column

input panel: support convenience top column

mirrors columnB (column bottom) but is placed on the top, i.e.,
columnT.

One can no conveniently set the following layout:

  +-------------------------------------+
  |                                     |
  |               columnT               |
  |                                     |
  +------------------+------------------+
  |                  |                  |
  |                  |                  |
  |   column1        |   column2        |
  |                  |                  |
  |                  |                  |
  +------------------+------------------+
  |                                     |
  |               columnB               |
  |                                     |
  +-------------------------------------+

(4 columns should work too, but we do not use that anywhere FWICT)

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>