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
    };
```

This enables most of the signals of GstRTSPClient

Only automatic bindings for now, which is enough to allow
implementing a simple rtsp-server example.

Depends on https://github.com/sdroege/gstreamer-sys/pull/8

Uses a new gir feature proposed at
https://github.com/gtk-rs/gir/pull/539 to make doc regeneration
easier.

Fixes https://github.com/sdroege/gstreamer-rs/pull/80

Generic methods for events, messages and queries can now be invoked from the concrete type.

Generic methods for events, messages and queries can now be invoked from the concrete type.

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`.

These should never ever be 0, and passing 0 to any of the functions
using them will result in assertions.

VideoFrames can always be converted to a VideoFrameRef if needed.

This is like VideoFrame, but can work on gst::BufferRefs and borrows
from it. VideoFrames can be converted into this by borrowing.

Not very useful, but will be useful for gst-plugin-rs

Values are passed owned to the callback

For consistency with the C API naming.

Should fix https://github.com/sdroege/gstreamer-rs/issues/68

Like all GObjects, mutation works through interior mutability.

This can be used to propagate an element error via a Result until the
place where it can be actually posted on an element.