Introduce project facets (parallel project types)

[thomasf]

This is a feature suggestion.

Currently eproject does hierarchical inheritence of project types with the exception that it does not allow to extend an project-type property but only replace it.

It would be great if multiple project types could be used at once.

An simple example would be an Django application which depends on coffee-script probably could benifit from being both project types at once.
At the moment I am merging most of my "web"-framework project types by hand and would probably use more project types and things like the recently added tasks if it were easier to mix and match.

Draft of simple functional requirements:

• Support for facets/parallel project types, possibly if they are identified at the same directory level.
• A project definition can be a main project type, project facet or both.
• There should be some control mechanism for which project facets an main project type can use.
• Easy toggling of facets/project types, possibly using the minibuffer/ido or similar.
• (maybe) a possibility to define which project facets that cannot be used at the same time.