GStreamer Redux

[vixalien]

GJS severly limits us in terms of what we can do with GStreamer, mainly due to it not supporting GStreamer signals very well. Here are some of the things we need to fix if/when that support lands:

• Correctly cache/cleanup downloaded files using downloadbuffer
• Correctly position the subtitle text to account for the player bar and preferred font by tweaking subtitleoverlay and textoverlay
• Use the Streaming API to correctly select which streams the user can play (based on the selected quality) instead of generating a new URL every time.