new function proto_tree_get_parent_tree()
it returns the parent tree of a given tree object

Change-Id: I191e0221651ffb5b26df761e9a25075aa0eb2ad3
Reviewed-on: https://code.wireshark.org/review/4954
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>