Add support for creating workspace private project descriptions

[laeubi]

Currently importing a project into eclipse requires the creation of a physical file name .project in the root of the project. this has several drawbacks for tools that automatically discover projects and import them for the user as it creates new files and possibly dirty their working tree. Other tools use a single folder for this purpose or don't require any permanent files in the working-tree itself. Even Eclipse has already such concept that is used when a project is located outside the workspace.

This now adds a new feature called "workspace private project" that only holds the basic information in the location file in the workspace directory, any additional information might be needed to restore by a tool that uses workspace private projects.

This is currently a draft what already works is that if one creates such a project is is open and can be used in eclipse, but there are some places that want to create / read the .project file (e.g. shutdown / restart).