Gitignore ".project" files from nested GAMA project in sources

[RoiArthurB]

Is your request related to a problem? Please describe.
While importing GAMA sources in an Eclipse project, it often happens to mistakely add every nested project at the same time (ending up with a mess or a new workspace).

Describe the improvement you'd like
Adding in the .gitignore the .project file detected by Eclipse as nested project to not have this behavior.

Additionnaly, those "nested projects" can have a placeholder file which can be rename as .project on start to have normal behavior with the model library while running GAMA in Eclipse mode.