[Stunner] Copy, Cut and Paste a selection on diagrams

Buttons for Copy, Cut, Past on the Toolbar
Workbench (project showcase)
Stunner toolbar (standalone showcase)
The logic on both cases should be shared, the buttons should trigger the same commands.

Keyboard shortcuts
Ctrl + C : copy
Ctrl + X : cut
Ctrl + V : paste
The logic should be the same as triggered by the buttons.

Additional information:

• When copy/cut/pasting the underlying graph instance must be updated by adding new nodes and, edges
  • New UUID must be re-generated for each new element
  • The bean instance related to each element must be "cloned" - TODO: Missing to check which some/all property values for the bean have to be cloned, at least, the element's name.
• Copy/cut/paste features should be implemented as core commands, this way the actions can be undone/redone and validations will be fired.
• If a single node is being copy/pasted/cut, its incoming/outgoing connectors will be NOT cloned
• The copy, cut, paste actions required firing diagram/bean validations, at least cardinality rule validations have to be run. If the pasted node is being placed inside a container, containment rules have to be checked as well.
• Copy/cut/paste have to work as well when multiple elements selected, so it implies multiple elements copy/pasting actions.
• [Optional] If a single node is being copy/pasted/cut, and it's inside some other parent node, the new node could be created as a child for the parent's one as well.
• The system will need to be able to gracefully handle a pasting that is illegal, maybe not allowing any other action until the user cancels the paste, using escape, or moves the paste to a legal location
• Copy/Cut/Paste to work between diagrams on the same desktop (Errai HTML Storage)