Improve new Sirius project template

In complement to https://github.com/eclipse/gemoc-studio-modeldebugging/pull/117
Many improvements may be done either in the current template or in new templates (this can be part of exercises given to student :wink: )

Some ideas:
- use EOpposite information to merge edges
- use containment information to display black diamond ends
- display containment as nested boxes instead of linked boxes
- wizard allowing to configure individually how a given containment relation must be displayed ( link, link + diamond, nested box)
- use information about existence of xtext editor (that is stored in the .dsl file) in order to automatically add xtext integration like the one in logo crash course (cf. https://github.com/dvojtise/mde-crashcourse-logo/blob/master/part3-mmfirst-solution/fr.inria.sed.logo.design/src/fr/inria/sed/logo/design/Services.java and in the odesign)
- add actions such as creation tool, drag and drop (in the palette and/or in popup in the diagram)
- other Sirius representations (tables ...)
- support of folding
- label based on _uriFragment_ when there is no _name_ attribute
- automatic change of the color of the container depending on the concrete type (instead of only gradient white to light grey)