NodeStyleDescription can only express square shapes

[pcdavid]

It should be possible to specify the width and height of nodes separately.
Currently we have a single expression which is expected to return a single number that is used for both dimensions: size = Size.of(computedSize, computedSize)

Options:

1. replace sizeComputationExpression with separate widthComputationExpression and heightComputationExpression: simple to understand and implement, makes it possible to specify one dimension of not the other, but breaks existing diagram modeles and adds yet more elements in an already crowded details view.
2. allow sizeComputationExpression to return full-blown Size objects, either as a string representation (aql:'150,' + self.computedHeight()) or with a new globally acessible service: aql:self.makeSize(150, self.computedHeight()). Feels cleaner but more costly to implement and brittle in the case of a string-based representation (need to define the syntax, to parse it properly).