Add support for copying/pasting between cells in grids.

The implementation should re-use and integrate with the existing Stunner mechanism (i.e. if there are toolbar buttons to copy/paste nodes they should be re-used to copy/paste cells when in the grid view. Copying a grid cell(s) should probably replace any node content on the virtual clipboard and vice-versa. In addition to toolbar buttons the feature should probably be also exposed on a context-menu and keyboard short-cuts...

Pasting a cell into another should only be possible where the type of cell is compatible. I.E. you cannot paste an expression cell (e.g. nested Decision Table on a ContextEntry) into a literal cell (e.g. Relation grid cell); however should be possible to past a whole grid (literal expression, decision table, etc) to replace an undefined grid.