Changes

[333] Migrate data used by SimpleELKLayoutTest

This commit just migrates the data used by SimpleELKLayoutTest without
any change.
It also seems that some diagrams have not been refreshed according to
previous modifications done on VSM file. All diagrams have been opened
(and so refreshed) for this commit.

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/333

[333] Add tests to reveal the edge routing problem

- 5 new representations have been added in tests data:
    - one with oblique edges,
    - one with rectilinear edges,
    - one with a mix of them and same number of oblique and rectilinear
edges,
    - one with a mix of them and more oblique edges,
    - and one with a mix of them more rectilinear edges.
- 5 tests corresponding to these representations have been added to
check the routing style before and after the arrange all with ELK.

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/333

[333] Store edge routing in parent

During the conversion from DDiagram to ELK Graph, the routing styles of
original Sirius DEdge is now stored on the common parent of edges in ELK
Graph.
This property can be overridden by defining the routing style directly
in the VSM.

The tests have also been adapted. Indeed, before this commit the routing
applied was always Rectilinear. Now, it is not the case. The existing
tests have been "duplicated", to have the previous behavior (by forcing
the "Rectilinear routing" in the VSM) and the new behavior for all
existing tests by considering the initial used routing style.

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/333

[333] Add tests to reveal the GMF style customization problem

Currently, a strange StringValueStyle is created on edge after a layout
with ELK, like this

<styles xmi:type="notation:StringValueStyle"
xmi:id="_b5DYgOqeEe6GK951-Ozudg" name="junctionPoints"
stringValue="()"/>

As consequence, the edge appears as customized (tabbar button "Reset
style properties to default", and same button in tab Appearance
Properties view, is enabled).

[333] Avoid to create useless StringValueStyle

Before this commit a StringValueStyle was created due to a bug in ELK
that creates an empty list of junction points.
This also causes, as side effect, an edge that seems to be "customized".

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/333

[333] Add migration participant to remove useless StringValueStyle

The representations created before the previous commit, that fixes the
unexpected creation of StringValueStyle, can contain some kind of "empty
StringValueStyle".
The goal of this commit is to remove them.

This commit also adds tests to check migration effect.

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/333