[INTERNAL] cleaned up AddProjectsToSessionWizard

Put many LOCS into dedicated methods as the finish method was very long
and unreadable.

Change-Id: I7fdc2499f34097dfc870b71de0f7389bf6660af3
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1894
Tested-by: Jenkins CI
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>