Session: Keep track of signal data locally

Session: Keep track of signal data locally

Currently, the Session class doesn't hold pointers to the signal
data itself but instead fetches the data on demand using the list
of signals it has. As the Session is providing the model in the
MVC paradigm, it owns the data and thus should keep track of it
directly. The signals are (in an ideal world) mere views on that
data, so querying them for the signal data isn't good style.