Clean code done for #359 by using new version of GMF/GEF #376
Description
Some code has been temporarily duplicated from GEF. This code will be APIs in GEF 3.20 version. The idea of this issue is to clean the code when Sirius will use a new version of GMF using GEF 3.20 (see GMF bug eclipse-gmf-runtime/gmf-runtime#62).