More validation for commands

[michael2402]

Currently, the command class does only few validation of the layer the command works for.

I suggest:

• Enforce that layer may not be null. Make command construction fail if it is.
• Check that all primitives of that command belong to the layer layer (for add/move/delete/... commands).
  • if plugins want to change this, we can provide them with the means to extend the commands. They have to take care of the problems that occur then, e.g. handle data integrity.