GStreamerSharp: Remove overkill debug output for each metadata tag
GStreamerSharp: Rework initialization to match the native backend

The native C GStreamer backend builds the pipeline during the "delayed
initialization" phase, which allows other extensions to be notified when
it is done. So we move the corresponding part from the Gst# PlayerEngine
constructor to the Initialize method.

This fixes the "Equalizer" menu item to be enabled when an equalizer is
available, and maybe some other things.
GStreamerSharp: Fix equalizer crashes

For the equalizer we're using the "equalizer-10bands" element, which
always has 10 bands and hard coded values for the band frequencies and
gain range. So we use these values directly, instead of going through
child elements, which causes crashes, and might not be possible anymore.

We could use the "equalizer-nbands" to be generic, but I don't see the
point right now.