Canonize tree branch order (Grace Joins etc)

[Hor911]

In plan tree view some branched (i.e. Grace Join inputs) are fully equivalent. Currently they are printed in random order, so the same plane can be rendered differently. It complicates plan comparison.

For such branches a "canonical" form with fixed (i.a. alphabetical) order should be implemented.

Some other nodes are already rendered with fixed order, no changes are required there

[pavelvelikhov]

I will need to double check if Grace Join inputs are really equivalent. We don't output them in random order right now, this order is defined by the cost function, and its currently not symmetric