Integrate the Boxed Expression Editor into the new DMN Editor properties panel making it possible to edit its cells

[ljmotta]

On the DMN Editor's Boxed Expression Editor, the properties panel should allow editing properties of Boxed Expression cells. When nothing is selected, the properties of the node containing the opened Boxed Expression should be displayed.

Currently, the selected cell of the Boxed Expression Editor isn't correctly exposed, this should be fixed before developing the properties panel.
Examples of incorrect behaviors:

• selecting a Boxed Invocation parameter will return an empty selectedObjectId.
• selecting a cell of a Boxed Context (e.g. its context entry name) and then selecting the boxed expression associated with it, will first return an undefined on the selectedObjectId, and then the correct id.

Tasks

• Correctly expose the selected Boxed Expression Editor cell.
• Implement the Boxed Expression Properties Panel enabling the user to edit the cells from the panel.