Add waitCondition closure to a content definition

Originally created by Chris Prior.

Add the ability to specify a waitCondition closure when defining a content item, to define what should be waited for before access the item.

Such as:

```
static content = {
    modal(wait: true, waitCondition: { it.displayed }) { $("#modal") } 
}
```

The wait property will be defaulted to "true" if waitCondition is supplied without wait.
