Implemented support for registering metamodels

Hi Jesus!

As promised a long time ago, here is my contribution for registering metamodels.

The plugin extends the UI to have a new popup menu entry when right-clicking on ecore files. 

At the moment, the menu entry appears in the root menu to avoid possible problems with the menu group (called "Model Facets") created in textresource2xmi project. In the future, I think that such menu group should be defined in other project so other plugins can incorporate new entries.