[Setools] Plotkit and Seaside 2.8

[Keith Hodges]

Dear Gjallarenes,

Our main reservation for moving to Seaside 2.8 was the suggestion that 
Plotkit was not supported.

It appears that the main reason Seaside 2.8 does not directly support 
plotkit is that the canvas html tag had been removed and placed in the 
separate addon package "HTML5".

So I tested plotkit with the latest Seaside 2.8, and the HTML5 package 
and all appears to be well.

Keith