Add bindings for CapsFeatures

[sdroege]

https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstCapsFeatures.html

Manually written (because miniobject), see caps.rs, sample.rs, etc.

Support needs to be added to caps.rs

[sdroege]

Looking into that now

[sdroege]

@fengalin maybe you can shortly review the serde implementation, but it's at least backwards compatible :)

[fengalin]

@sdroege, yes this is on my TODO list!

[sdroege]

Thanks! Did you also get my mail from earlier this week?

[fengalin]

Yes, thanks. I'll register ASAP.

[fengalin]

@fengalin maybe you can shortly review the serde implementation

LGTM :)

[sdroege]

Thanks :)