less if-nesting in ThumnailView

Easier to read.

Change-Id: I1ceb551d8badffb211b1f444b61375a03cac1348
Signed-off-by: Rodolfo Ribeiro Gomes <rodolforg@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/6643
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>