Improve platform selection in the ChannelNode

* Adds a function for updating platform when synchronizing the
ChannelNode
* Simplifies the getting/passing of the platform information by
shortening the prototype chain
Add multiplatform support related to the Executer creation

Adds multiplatform support into the ProcessNode related to the Executer
creation.
Fix a bug that prevents setting platform information in ChannelNode
Add a CLProgram prototype stub

Adds a CLProgram prototype stub code (currently identical with
the SingleProgram). The WebCL operators will utilise CLProgram in
processing.
Add WebCL platform support into createProgram

Xflow.createProgram will now create a CLProgram instance if
an operatorList utilises the WebCL platform.