@ConfigProperty code action when property is not set

[angelozerr]

Once #146 will be implemented, we could provide several code actions.

Given this @ConfigProperty:

public class InventoryConfig {

    @ConfigProperty(name="port")
    private int port;

}

port will be higlighted as an error, and we could provide the following code actions:

• configure in Java => it will generate the , defaultValue="${0}" attribute
• configure in application.properties (and other existing properties) => it will generate the port = property

For yaml files, I fear that it's not possible to do that because we don't have a Yaml parser with location.