Kernel: Fixed dangling icons and background color in TreeTableViewers

- Icons are now removed, if a row does not contains an element anymore.

- Fixed a formating issue which prevented displaying background color on
some systems.

Issue-Ref: 4147
Issue-Url: af3#4147

Signed-off-by: Andreas Bayha <bayha@fortiss.org>
1 job for !148 with 4147
detached
Status Name Job ID Coverage
  Test
failed downstream-job