German translations and minor clean-up in ScHeaderControls

Some methods properly defined as const methods

Change-Id: I59bc371bbb3adc6df2b96b82a0968a8693622ff8
Signed-off-by: Rodolfo Ribeiro Gomes <rodolforg@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/6537
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>