scene::Session => scene::Application

[Alan Griffiths]

With the current review of "trust session" it is clear that "Session" is
a bad name for what the scene::Session class represents.

I suggest that we rename it to "Application" as that seems to better
reflect its role.

Clearly that has the potential to break some downstream projects - but
that could be mitigated by the temporary introduction of some typedefs
and forwarding functions to give us API compatibility while we update
those projects.

Any opinions on the desirability of doing this rename?