Added rename

So I added a rename function into gobby.
It mostly works, except that there is a signal needed, to let know when a node has been renamed, so we can change the TitleBar and TabLabel text accordingly.
One way would be to connect a signal in SessionView.cpp, and have inf-browser.c send the signal through the corresponding InfSession. But there might be some other better way.
Should be just called `SignalTitleChanged`, I think. Calling it `title` makes it consistent with `get_title()`, and the whole class handles a session, so the session can be omitted.