Simplify launcher for the official example

## Motivation

Currently, the official example (K3FSm, SigPml, etc ) uses a shared launchconfiguration (saved in a .launch file) in order to preprare the launch of the second eclipse (Modeling workbench) installed with the language.

These launch configuration are heterogeneous (name of the launch conf and name of the workspace folder)

We need to help GEMOC user by having a clear naming for this.

## Proposed Solution

I propose a 2 step change:
- first use in all our example the same naming convention in the saved launch configuration file
- second implement a UI action that directly create and launch the second eclipse (it would internally create the same launch conf,) so we don't have to save these launch conf in git anymore

## Proposed Implementation

### Step 1:
Use the following naming convention:
- `Launch GEMOC Modeling Workbench for ` for the launch conf name
- `${workspace_loc}/../gemoc-modeling-workbench` for the worspace folder name

Also remove the unused mwe2 xtext specific launchers since these action are already directly accessible with a right click on the .xtext or the .mwe2 files or project

### Step 2:

Add an action (for example on the xdsml project) allowing to run/debug the language by:
- if not existing launch conf exist, create one
- run the launch conf

proposed name for the action (to go with the existing other in the menu cf. above screenshot): `GEMOC Modeling Workbench`