Don't update parent if the old and the new are identical.

This fixes the reorder issue when resize_obj was occuring. The resized
object was inserted at the end of the children list of the parent.
Sequence add: add name validity

We check now that the name is not NULL and that doesn't name an existing
sequence.
Update TODO
Context: remove hash table for sequences
Prettify graphviz
Revert "add HINT_ALIGN_SET -1 for each widget by default"

This reverts commit 9d9e15d764b2143629549fb2ab88052d7bb2a788.

You don't want it on buttons. I reverted it because I don't have the
force to think which widgets need it. You have time on Sunday.
Make property hardly dependent on op.

  There are two functions to add property now:
  - prop_create_empty_for_op()
    so need to add values with prop_value_append
  - prop_create_for_op() which creates property with values.

  Going to remove first one.