Tutorial on how to write GStreamer plugins in Rust

This tutorial is for the gst-plugin-tutorial plugin. This plugin provides 4 features. There are separate turoials for rgb2gray and sinesrc feature for now and there will be more in future.

1. Part 1: rgb2gray - A Video Filter for converting RGB to grayscale
2. Part 2: sinesrc - A raw audio sine wave source
3. Part 3: identity
4. Part 4: progressBin - Prints progress information. Also showcases how to implement an enum-property on a plugin.