[generator] Revert change to use auto-properties in EventArgs classes.

[jpobst]

In #726, properties in generated EventArgs classes were changed from regular properties to use auto-properties:

From:

int param1;
public int Param1 {
    get { return param1; }
}

To:

public int Param1 { get; }

However this is technically a breaking change, since users could have hand written code that references the backing field. Although the usage is likely rare, we have decided to revert the auto-property change for compatibility reasons.