[RELEASE] disabled ConcurrentUndo feature

This feature can enter an infinite loop and so crash/stall the GUI
forever.

Change-Id: Ib9d6b07b6a0498251a6fc1807ef7408a3f7f83f7
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1992
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>