'Add missing case statements' uses alphabetical order and not declaration order

This was originally reported [here](https://bugs.eclipse.org/bugs/show_bug.cgi?id=546984) but I think this might be a useful feature or worth a discussion at least.

-------------------
With JDT included in Eclipse 19-06 M1, using the quick action 'Add missing case statements' will generate unordered case statements.

For example, with this enum:
```
public enum ENodeIndex {
TIP, LEFT, MIDDLE, RIGHT
}
```

The generated code is:

```
switch (index) {
case LEFT:
break;
case MIDDLE:
break;
case RIGHT:
break;
case TIP:
break;
default:
break;
}
```

I think it would be better to keep the enum order.

----

Start at the Eclipse JDT UI implementation of the “Add missing case statements” quick action and inspect how enum constants are ordered. Add coverage using the ENodeIndex example, and verify that generated cases retain the declaration order TIP, LEFT, MIDDLE, RIGHT.

Written by the indexing model from the issue text.