Hi, implemented different width for the (mult-edge) landing (as no much idea how other features should be done

)
A new Property 'Width of Landing' is added
It is a list of width which set the width of different 'edge' of a Landing
- this is for 2nd edge and after, 1st edge follow original Width property
- 'default' setting in original Width property would be used if a '0' (zero) setting is made for any 'edge'
Below ArchStairs.py is for 0.18_pre
Version for 0.17 is available if anyone is intertested)
It is implemented with App::PropertyFloatList ... as App::PropertyLengthList is not yet available
See if there is any comments before a PR is being made.
Thanks.

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