Support for external Metafacture distribution and plugins

[cboehme]

Metafacture seems to be baked into the IDE at the moment. It would be handy to be able to use an external Metafacture distribution so that Metafacture can be updated independently of the IDE.

This would also allow users to use the plugin mechanism in Metafacture.

[fsteeg]

This should probably be a project-specific setting, since different projects depend on different Metafacture versions. See also #46.