adaptivedemux: replace ghostpad with a standard pad

adaptivedemux: replace ghostpad with a standard pad

Handling the ghostpad and its internal pad was causing more issues
than helping because of their coupled activation/deactivation
actions.

As we have to install custom chain,event and query functions it is
better to use a floating sink pad internally in the demuxer and just
use those pad functions to push through a standard pad in the demuxer

https://bugzilla.gnome.org/show_bug.cgi?id=757951