GStreamer Video - Port over MSE stubs from AAMP code

[jmgasper]

Problem

We did some work previously to generate some MSE APIs using the old AAMP player. You can find the requirements for that work here: #285

Currently, we are building out the HTML5 video element and the MSE APIs without using AAMP, so we need to port the code from the previous branch over to the new codebase, stripping all the AAMP functionality and replacing it with the GStreamer integration, while keeping the same MSE API.

You can find the MSE / AAMP code here:

https://github.com/topcoderinc/pxCore/tree/_topcoder_mse

Fix for this challenge

For this challenge we are going to implement the exact same requirements as detailed in #285, but using GStreamer instead of AAMP. We will not use AAMP at all, for any requirements. You can use the stubbed code from the branch above, but note that it will have to be edited to avoid any reference to AAMP, including the folder path where the code is placed (mse would be a good folder name instead of aamp)

We want the video playback screen updated as well to use the MSE API.