Update of a CMD element in the Workflow Designer

Currently, when an element with command line tool is created in the Workflow Designer, it can only be edited by selecting the "Edit" item in the context menu (for the element) on the Palette.

Also, when the element is modified, all current instances of the element disappear from the Scene.

The following modifications are required:

1. An "Edit" item should be added to the context menu of the element on the Scene.
2. When the element has been edited, all element instances on the Scene should be updated, if it is possible (i.e. the selected values of parameters should not be modified). Otherwise the user should be informed about the reason of the problem and asked, if he would like to apply or cancel the changes.