[gstreamer] Replace appsrc via uri handler

Qt uses appsrc:// to read from QIODevice, which has several issues:

• threading and ownership of the QIODevice instance isn't clear
• going through signals adds overhead
• attaching the QIODevice to the appsrc element is rather convoluted.
• the same QIODevice can only be used in one GstAppSrc element

Replacing the GstAppSrc element with a uri handler reduces complexity and overhead, while also allowing us to use the same (seekable) QIODevice in multiple locations (players / discoverers)