GStreamerSharp: port to GTK3

Even though GStreamerSharp backend doesn't strictly depend on GTK at
compile time, it uses GDK at runtime to get the handle of a Window
in order to tell GStreamer where to render a video. To do this,
it uses a P/Invoke to an unmanaged library which was still pointing
to GTKv2.0 instead of GTKv3.0. This was causing the following errors:

(Nereid:16860): GLib-GObject-WARNING **: cannot register existing type `GdkWindow'

(Nereid:16860): Gdk-WARNING **: /build/buildd/gtk+2.0-2.24.17/gdk/x11/gdkdrawable-x11.c:952 drawable is not a pixmap or window

We leave a couple of FIXMEs for Windows/Mac platforms.

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>
GStreamerSharp: make video playback work properly again

VideoManager initialization being commented out meant that video
playback was working, but not in the proper place (a new window
would pop-up instead of playing the video inside the NowPlaying
area). This was caused by the bad usage of the Gst# method
Gst.Bin.GetByInterface(), which receives a GType, but was getting
a Type of an interface that corresponds to the GInterface type.
The way to do it is using the -Adapter class that maps to this
GInterface.

We commented out other lines that cause the player to freeze/hang
altogether (probably related to some threading deadlock). Commenting
them out means that GStreamerSharp still has a bug with regards to
playing a second video while a first one is already being played,
but the latter bug should be easier to solve, for now, than the
freezes.