[GUI] add option to create a session nickname

Also corrected an defect in the Personalization Page. When pressing the
"Defaults" button the resulting changes were immediately written to the
store.

Change-Id: I2e7ba8432cd8dc61d60d5192a7c3e9a9badf7b2d
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1616
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>