Duplicate object name if second created during creation of first

[Juan Pablo Caram]

How to reproduce:

1. Start a (long) process that creates an object (paint a polygon).
2. Repeat 1 before 1 is complete.

Apparently the name is computed at the beginning of the creation process but not registered with the application until it completes, therefore multiple processes can compute the same name if the others haven't completed.

For the above to be possible, the processes must be in different threads, or the computation of the name and the registration of the new object happen in different threads.

[Juan Pablo Caram]

Moved object name collision detection from new_object() to ObjectCollection.append().