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
};
```
Concrete events and messages share common attributes which can be factorized in generic builder `struct`s. This reduces noise in the concrete implementations.
Implement `deref` for concrete derivatives of `Event`, `Message` and `Query`. This allows accessing generic methods such as `is_sticky` for events, `get_seqnum` for messages or `is_serialized` for queries and `get_structure` for all.
These methods are commented out in order to maintain consistency between messages with and without arguments. It will be restored and conditioned to feature "v1_14" when GStreamer v1.14 is released.
Building an argument-less message such as eos yields an assertion failure due to the inner structure being null. The short term solution consists in checking that the inner `structure` is not `null` before attempting to insert `other_fields`. The consequence is that `others_fields` defined for argument-less messages will be ignored. A correction will be applied when GStreamer 1.14 is released thank to the introduction of `gst_message_writable_structure` (see https://bugzilla.gnome.org/show_bug.cgi?id=792928). Due to the dependency on GStreamer 1.14, the correction will be only available under the activation of a feature "v1_14".
Events are not affected as the build method the availability of `gst_event_writable_structure` and this function "will never return NULL". However, we can avoid a `structure` allocation for argument-less messages without `other_fields`.
