Add support for new MDK decoder

This adds support for a new decoder based on the MDK-SDK which uses FFmpeg to do most of the work. It's the successor to the QtAV decoder which is still supported but development on it has stopped so the MDK decoder will be the one to use from now on.

The decoder will be dynamically loaded if it is installed.

There is a new script that will download the full SDK and extract the libraries for one of the supported architectures (amd64, arm64, armhf) and install them where MythQML can find them and dynamically load them if needed.

Examples of how to run the script

# install amd64 libraries (default if no option is given)
sudo /usr/share/mythtv/qml/Scripts/installMDK.sh

# install arm64 libraries
sudo /usr/share/mythtv/qml/Scripts/installMDK.sh arm64

# install armhf libraries
sudo /usr/share/mythtv/qml/Scripts/installMDK.sh armhf