event/message: provide mutable API

See the following snippet changing caps from a pad probe:

if let Some(gst::PadProbeData::Event(ref mut event)) = &mut info.data {
    if let gst::EventView::Caps(caps) = event.view() {
	let mut updated_caps = caps.caps_owned();
	{
	    let caps = updated_caps.make_mut();
	    let s = caps.structure_mut(0).unwrap();
	    s.remove_field("framerate");
	}

	let updated_event = event.make_mut();
	let event_s = updated_event.structure_mut();
	event_s.set("caps", updated_caps);
    }
}

It's not that easy to figure out, so we could provide better API to make it easier:

• event.view_mut() as we already have QueryViewMut
• provide setters on the event so we don't have to modify the GstStructure