This PR tries to extend DASH support in a clean way to also support
built-in implementations as with SPMC. Besides a minor fix, there are
three main pieces:
Finish off partial support allowing to identify the application e.g.
Kodi, SPMC, etc.
Add hidden settings for built-in and addon-based DASH support. Visible
settings depend on these.
Add a startup service using application and version info to initialize
the hidden DASH settings
This approach has the advantage of handling downgrades, etc. and
populating DASH settings correctly without having to first run the
addon. I've tested this on Android with both SPMC 16.5 and Kodi 17b6
without problems.
To make it easier for others to try out I packaged a "5.3.7" version
here: plugin.video.youtube-5.3.7-spmc.zip
