Reconsider Command.isPushable

[JanKoehnlein]

We have implemented exceptional handling for SetBoundsCommands (isPushable returns false). The reason is that they are automatically fired and as such should be neutral to Undo/Redo. We should rather push them to the stack but

• undo them and the next command on an undo action
• not let them clear the redo stack

[JanKoehnlein]

We now have the notion of a system command. An example is the SetBoundsCommand. These commands should not be individually undoabl/redoable, and they should not break the chain of commands in an undo redo scenario. This means

• when the redo stack isn't empty when a system command is executed it is pushed to the offStack. If after that
  • a real command is executed, then the offStack commands are pushed to the undo stack before the real one
  • a further undo happens, then the offStack commands are undone and the offStack is cleared
  • a redo happens, the offStack is undone and cleard as well as they should not break the chain of commands on the redo stack On each undo, commands from the undo stack are undone and pushed to the redo stack until/including the first non-system command. On each redo, commands from the redo stack are redone and pushed to the undo stack, including leading and trailing system commands surrounding at most one non-system command.

I've added a few tests for this.