Update to XText 2.13 to retain comments after OML Normalization

[NicolasRouquette]

In XText 2.12, the resolution of #155 results in loosing all comments when a user invokes the OML Normalization action from the OML editor.

To avoid this, the suggestion is to upgrade to XText 2.13 to use a new mechanism that should enable retaining the current syntactic information in the OML Normalization. See eclipse-xtext/xtext#2592

[NicolasRouquette]

There are several updates to consider:

1. Oxygen => Oxygen1.a
2. XText 2.12 => XText 2.13
3. Sirius 5.0.1 => Sirius 5.1.0
4. Acceleo 3.7.1 => Acceleo 3.7.2