Correct message about a custom element deletion from the scene

See the screenshot of the current message that appears when a custom element with external tool is edited and input/output data or parameters are modified.

The message should be changed to:

You've changed the element structure (input data, parameters, or output data).

If you apply the changes, all elements of this type will be removed from the scene. You can then add a new such element to the scene by dragging it from the "Custom Elements with External Tools" group of the "Elements" palette.

Would you like to apply the changes?

The buttons should be renamed to "Apply", "Reset to initial" и "Cancel". Location of the buttons should be modified as well: put the "Reset to initial" button into the middle.