Upstream gst shim enhancements

[nlowe]

We've made a few changes, we should upstream them.

Already Implemented

• [x] Windows Compatibility: https://github.com/notedit/gst/pull/13
• [x] float64 support for (*Element).SetObject(...) https://github.com/notedit/gst/pull/14
• [x] (*Element).QueryPosition() / (*Element).QueryDuration() https://github.com/notedit/gst/pull/15
• [x] Support for extracting fields from message structs https://github.com/notedit/gst/pull/16

Planned

• [ ] More control over log spam #10