[DMN Editor] Move Editor controls inline

Expression Editors have a row of controls to add row, column, change Hit Policy etc.

These all need moving "inline" into the editor itself. The content of the new context menu will be driven by the cell in the Expression grid that was (single) clicked. Double click continues to edit the cell.

New requirements

• Context menu (single right-click) to provide the ability to cut/copy/paste/insert (row/column)/delete (row/column)/delete cell value
• Double Single left-click to edit a cells content (this could be an TextBox or a ListBox or a more sophisticated editor).
• Context menu entries are reflected in the state of the toolbar (menubar, whatever) in the workbench.

This means the "Undefined Expression selector" (DROOLS-2296), "Context Editor" (DROOLS-2297) and "Relation Editor" (DROOLS-2299) will need refactoring.