Implement `bootstrap-gstreamer` for all platforms

This change makes it so that the Platform classes can now handle
installing GStreamer dependencies and properly setting up the
environment including when cross-compiling. For Windows and Linux
is now installed into `target/dependencies/gstreamer` when not installed
system-wide. In addition:

1. Creating and moving existing environment path append helpers to
   `util.py`.
2. Combining the `set_run_env` and `build_dev` functions and moving
   some outside code into them so that it can be shared. Now code that
   used to call `set_run_env` calls `build_dev` and then
   `os.environ.update(...)`.
3. Adding Python typing information in many places.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

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