Melange file in src folder may lead to corrupt manifest and incorrect deployment

The fact that currently the melange file is contained by a java source folder has several side effects.

1/ the content of src folder is duplicated in the target folder (usually *bin* folder) thus the gemoc builder that updates the plugin.xml and manifest is launched 2 times

Launching 2 times the builder can have 2 effects,
* the plugin.xml may randomly point to the src or bin version (which is a pain when managing the file in git)
* the manifest writer seems to not be thread safe and lead to corrupt manifest file (see bug such as https://github.com/diverse-project/melange/issues/105)

2/ when deploying the plugin the path to the melange file is not stable, src folder is not supposed to exist in the resulting jar, bin folder doesn't exist as its content is placed at the root of the jar/zip. Thus the path in the plugin.xml is wrong when the plugin is deployed (the path is accidentally correct when using 2 eclipses). It would be safer to put the melange file in a dedicated folder (such as *model*) and make sure this folder is declared in the *bin.includes* entry of *build.properties* file. (ie. similarly to the .ecore, .genmodel, etc)