Previously, the properties only accept a single DIRECTION enum value. It wasn't possible to pass a string related to
the DirectionValue type, it generated a TS error.
In addition, it is now possible to pass several values, as this was possible in mxGraph. This is what the Orthogonal
story requires for example.

For backward compatibility, the implementation still support the former mxGraph configuration where the directions were
passed as a concatenated string. This ensures that the mxGraph XML models can be correctly loaded.

About 20 tests have been added to validate the implementation.