Changes

[343] Refactor - Extract trace debug in a specific class

This commit also makes some minor changes:
- Usage of ELK Options constant ID, instead of hard-coded values
- Display of the message dialog is now the responsibility of the caller

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/343

[343] Add a property system to allow to choose the export folder

It is now possible to launch the Eclipse with the option
"-Dorg.eclipse.sirius.diagram.elk.targetFolderPath=myPath" to select the
folder where the diagrams are exported (through explicit action or
through the trace debug options).

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/343

[343] Add a new action to export diagram as ELK Text format

This commit provides a new action named "Export diagram as ELK Text", as
for "Export diagram as ELK Graph", but that exports the current diagram
in a file using the ELK Text format (format used, for example, in ELK
Demonstrators [1]).

[1] https://rtsys.informatik.uni-kiel.de/elklive/elkgraph.html

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/343

[343] Extend ELK graph traces options

- Add the textual mode as an option for the debug traces. It is enabled
by default.
- Add the xmi mode as an option for the debug traces. It was the only
choice before (enabled by default as soon as debug is enabled).

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/343

[343] Clean code for ELK export.

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/343

[343] Refresh workspace after export actions

If the exported file is in the workspace, this last is refreshed (with
the action launched in a WorkspaceModifyOperation to avoid collision
problem).
The workspace's refresh is not launched during the creation of the files
during the arrange all, to avoid potential side effects. In this case,
the refresh will still have to be done manually.

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/343

[343] Add automatic tests corresponding to these enhancements

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/343

[343] Add corresponding documentation

This documentation does not concern only the enhancements. Indeed, the
existing capabilities were not documented before.

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/343

[doc] Update URLs still pointing to the old website address

Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>

[411] Enable pin tabbar action on edges

This commit:
- enables the "Pin status" button on edges,
- automatically marks "moved" edge as pinned (if corresponding
preference is true),
- updates the documentation,
- adds additional tests in existing tests classes concerning pinned
edges.

Bug: https://github.com/eclipse-sirius/sirius-desktop/issues/411