[3763] Split the SelectionDialogDescription to prepare the Tree support

[florianbarbin]

The SelectionDialogDescription will now just hold the message information
The SelectionDialogDescription will now have a
selectionDialogTreeDescription to make it possible for the specifier to display the selectable elements as a tree.
The selectionDialogTreeDescription will be relatively simple at first time.

Bug: #3763

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The current SelectionDialog behavior is still the same.

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[sbegaudeau]

Most of the code looks good, I'll rebase it and fix the tiny issues detected here and there (it should mostly be conflicts with master)

[sbegaudeau]

Can't we use a flatMap somewhere?

[sbegaudeau]

I'll wait for the build and merge it