Provide a unique DSL nature+extension point

For each DSL developped with the GEMOC Studio, we need to rely on some extension point to be able to automatically detect and use all provided DSLs in the modeling workbench.

At the moment, this is achieved through different Eclipse natures: one for sequential xDSMLs, and one for concurrent ones. Each of these natures has its own extension point with a similar name.

Since now we rely on a generic format to declare and configure DSLs (the DSL file), it would be logical to replace existing extension points by a unique extension point "GEMOC DSL", and existing natures by a unique nature "GEMOC DSL Project". The nature would automatically detect `.dsl` files present in the project, and generate the corresponding "GEMOC DSL" extension, similarly to our existing natures.

At the same time, we can discuss replacing the term "xDSML" by simply "DSL" throughout the GEMOC Studio, to be more aligned with the "DSL file".