Support Spacer widget inside WrapSuper

[ekuleshov]

I have some layout challenge which might be possible to resolve if WrapSupper supported the Spacer widget as a child or something similar to it.

Basically I'm trying to implement layout like:

| <child1> | .... empty space .... | <child2> | ... | <childN> |

And the idea is that I'd like these children would fill in the available width, and the <empty space> would shrink as available width decreases. Then if child2..N don't fit on the available width, they get pushed to the next row (and to rows after that). In that case the <empty space> would fill in the rest of row 1.

Additionally it would be great to be able to make the 2nd row (when it is not fully filled in) to be right aligned. Assuming that the <empty space> would push the trailing <child1> (or multiple trailing children) to the left.