Added clipboard (cut, copy and paste)

Sorry for the multiple commits. I needed some small refactors here and there.

Copy mechanism is based on the following:
1) User executes copy (or cut) action.
2) The editor asks for the active view (for now, active view is that with the keyboard focus)
3) Once it has it, editor asks to the view for the schema object it want to add to the clipboard (if any).

Paste mechanism is based on the following:
1) User executes paste action
2) Editor asks retrieves the content of the clipboard (usually, a schema object), and see if there is a PasteListener that processes the content.
3) If yes, the model is updated with the pasted schema object.