[DMN Designer] Not expected results of commands when multiple cells selected

When multiple cells of the decision table are selected, then the behavior of inline context menu can be confusing for the user.
Few scenarios:

1. Select multiple cells, invoke context menu, select delete rule -> just one deleted
2. Select multiple cells from different columns, invoke context menu, select delete -> just one column deleted
3. Select both input and output cells, context menu entries are established according to the last click
4. Select continuous block of cells, invoke context menu, selec insert rule below -> rule is not inserted below the most bottom cell of the block
5. ...

Due to this, we should disable the context menu entries which could bring confusion for cases when multiple cells are selected