Memoize diagram representation

[mcharfadi]

When selection a node, the workbench is rerender and all the representations.

This is the result of profiling after changing the selection several times by clicking on nodes.
On a diagram with more than 1k nodes, the longest re renders are taking around 350ms

The same profiling but with DiagramRepresentation and ExplorerView memoized
The longest re re render (triggered by the re render of the Workbench) does not exceed 50ms.