Add TermSession class to represent a terminal session

Add TermSession class to represent a terminal session

At the moment, the assumption that each terminal activity contains only
one EmulatorView and hosts only one terminal session is fairly deeply
embedded into the code.  To help break this assumption, we introduce a
new TermSession class, which holds the TerminalEmulator and
TranscriptScreen for a session, and handles the launching and watching
of the process and the setup of the I/O streams.