[DMN Designer] Add visual indicators for imported Nodes

Imported models are read-only. We need to show some form of visual indicator for both Nodes (Graph View) and Boxed Expressions (Grid View). Please see the attachments for proposals for the Grid View.

Design Resolution:

• Option C: https://issues.jboss.org/secure/attachment/12449764/read-only-gridC2.png
• Keeping the hover highlight per row cell as is, because when those elements are clicked the Property panel is updated/reflects.
• Use lock icon for canvas nodes: In the components list and in the graph nodes, as illustrated in: https://issues.jboss.org/secure/attachment/12449758/Screen%20Shot%202019-04-17%20at%2011.55.39%20AM.png