François Laignel authored Feb 15, 2018 and Sebastian Dröge committed Feb 16, 2018

Allow instantiating and dereferencing concrete queries. The motivation
for this proposal is to allow the following usability enhancements:

- Concrete queries mutability guaranteed by the borrow checker,
including for generic functions:
``` rust
    let mut p = Query::new_position(::Format::Time);
    p.get_mut_structure().set("check_mut", &true);
```

- Concrete queries functions available in place:
``` rust
    let mut q = gst::Query::new_duration(gst::Format::Time);
    let duration = if pipeline.query(&mut q) {
        Some(q.get_result())
    } else {
        None
    };
```