Rationalize selection element name handling.

Rationalize selection element name handling.

The name of SelectionTreeElement is now a plain std::string that manages
its own memory instead of pointing to memory managed somewhere else.
The original design has been a pain to maintain at times, as the memory
for the name easily gets freed, leaving a dangling pointer.

Change-Id: I7b975c67ce9eee7f981128e29d8d9dd75fc70c89