Bug: Motion steps are marked as "Done" even when motion was not issued

[schuetzepaul]

Since in #191 we had to return a signal (in this case we use motion_finished) in order to be able to continue with the assembly even in case of stage issues (i.e. reaching the stage limits), the AssemblyAssemblyV2View now recognises such a case as completion of an assembly step.

A signal such as motion_aborted would be more appropriate and could be dealt with by not marking the step as "Done". However, motion_finished is at the moment also required by many other receivers in order to re-enable buttons or widgets.

I would nevertheless attempt a solution with a motion_aborted signal, but we need to make sure to also connect this to all corresponding slots.