Return the tree ID when registering a file

[galou]

Return the attribute "main_tree_to_execute" or the ID of the single behavior-tree.

I was playing around with registerBehaviorTreeFromFile() and createTree and realized that I was missing the ID of the tree to execute. This commit solves this.

[facontidavide]

I think this is useful, but being an API change I need to wait for the version 5 release