It means that there were no changes.

Fixes https://github.com/sdroege/gstreamer-rs/issues/11

It's immutable.

It's useless and not used.

Also fix structure ownership for the cache data API

Also add a getter for the description of a debug category

The origin in protection events is optional, the stream id in
stream-start events is not.

Also return a segment reference instead of a segment copy when parsing a
segment event.

It can contain no value.

Also use &ToSendValue instead of an actual value for building the
message.

Also ::copy_deep() can fail, return an Option<_>

The implementation for Option<_> is doing exactly what we did manually
here.