Improve 'bad names' usability

[tznind]

If you edit Name property and set it to blank and confirm the choice then you get a quite ugly dialog and the value reverts:

If you try to set Name to the name of another field it will add a number to the end:

• Add 2 buttons
• First will be called button1
• Second will be called button2
• Attempt to rename button2 to button1

Expected:
Some kind of error message
Actual:
Button is renamed to button12

[dodexahedron]

Maybe a prompt with a suggested action (such as the action it would currently take), and a cancel, which aborts, returns things to how they were, and maybe event presents them with the menu they were previously looking at (if that's doable).