[GUI] allow the start of host only sessions

This might be needed for future features such as session join on demand
requests etc.

Changed also the layout of the wizard page. The buddy label is not
needed as it is already explained by the page description what the user
will select in the table.

Change-Id: Ifda9098036a158cc0d03db0e5daf99b09110cb3d
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1110
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>