shell: Enhance the basic random positioning algorithm

Place the window in a random position on the output where the first seat with
a pointer is. When calculating the random position limit the range to the
area that would ensure that the whole surface is visible. If the surface is
larger than the output then the surface is placed at the origin of the
output.

This change is based on the good work of Scott Moreau <oreaus@gmail.com>
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