[GUI] changed layout of ColorChooserWizardPage

The ColorChooser composite is now centered in that page.

Change-Id: Ifbbc2ffa66f15663eff9246d57f5078fee4fc556
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1184
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>