Revise ownership of terminal instances

Currently the Terminal objects are parented to the base splitter

When they are destroyed the session reacts to that and removes them from its internal map

This is rather complex and causes a crash when closing the app

Instead have the session own the terminals (and manage them using unique_ptr)

This is complicated by the fact that the part instance deletes itself when the terminal is exited by the user

In that case notify the session that the terminal wants to be deleted and clean up accordingly

BUG: 456917