Proposal: instantiate {Event,Message,Query}s from the target type

Instantiating an {Event,Message,Query}, implies calling the C-alike new_... functions on the generic type. Ex.:

let stream_start_evt = gst::Event::new_stream_start(&stream_id).build();

For Events and Messages, we actually get a builder which allows adding optional data.

In Rust, we could expect the following instead:

• For mandatory fields only:

let stream_start_evt = gst::event::StreamStart::new(&stream_id);

• For mandatory and optional fields:

let stream_start_evt = gst::event::StreamStartBuilder::new(&stream_id)
    .group_id(gst::GroupId::next())
    .build();