[Seaside] Seaside 2.13

[Avi Bryant]

Hi all,

A new release of the experimental 2.1 version of Seaside is now available
at http://beta4.com/seaside2/downloads/Seaside2.13.st.gz .

Although this uses the same StateTree mechanism for managing backtrackable
state as previous 2.1 releases, the interface to it has changed.  Rather
than directly sending #at: and #at:put: messages to a StateTree instance,
components can now obtain a StateHolder from the session (through
WASession>>stateHolder).  This has the same protocol as ValueHolder, but
will update its #contents properly during backtracking.

Hopefully this is enough more convenient than the direct StateTree usage
that the custom class definition template and auto-generated accessors
in earlier versions of 2.1 are now unnecessary.

Feedback desired, as always.

Avi