1. ElementImpl trait requires GstObjectImpl
2. gst logging macros are no longer globals prefixed with gst_, they
live inside gst:: instead
3. element is not longer passed around in many places, it can be
accessed as self.obj() or self.instance()
4. query_default is now a part of gst::Pad and takes the pad as an argument
5. some constructors were changed to use from_$type()
6. query.view_mut() returns QueryViewMut
7. ElementFactory::make now returns a builder that we have to .build()
There are some extra cleanups as well:
1. spurious 'mut' and '&' are removed, a lot of that can be infered or
were turned into a dereference by the compiler anyway
2. !bla.is_ok() are now bla.is_err()
3. some unneeded imports were removed
