Add seed selection to create world menu.

[Ekdohibs]

No description provided.

[sfan5]

There are a lot more options when creating a world (dungoens, jungles, etc.), I think the whole menu should be
redesigned and all of this should be added at once.

PilzAdam's comment on #729 (comment)

[kahrl]

copied from #minetest-dev

<kahrl> nore, I don't think your commit works
<kahrl> you can't rely on the user starting the world immediately after creating it
<kahrl> try creating two worlds with seeds 1 and 2 and then starting the first world
<kahrl> btw, is there a way of doing this that doesn't punish users whose copy+paste doesn't work in irrlicht?

[kahrl]

hmmmm is working on an extended version of this.

[kwolekr]

err, sorry I did this already