GStreamerSharp: adjust GDKv3 lib name in Windows

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>
GStreamerSharp: fix build

We have a DllImport checker in the build that makes sure that we're
mapping all P/Invokes to proper libraries via a .dll.config file,
like explained in:

http://www.mono-project.com/Interop_with_Native_Libraries#Library_Names
GStreamerSharp: protect call to event delegates

This is a good practice to avoid potential race conditions, that
is already used in other events, and is best explained in:

http://www.mono-project.com/Gendarme.Rules.Concurrency#ProtectCallToEventDelegatesRule

or

https://github.com/spouliot/gendarme/wiki/Gendarme.Rules.Concurrency.ProtectCallToEventDelegatesRule%282.10%29
GStreamerSharp: prepare overlay the soonest possible if it's a video

There was still a small (but noticeable) time window between the start
of a video playback and the mapping of its rendering box to the
NowPlaying window. Doing it just after the Play() command is sent to
engine is the soonest possible it can be done to avoid this visual
glitch.
GStreamerSharp: prevent potential NRE in FindOverlay()
GStreamerSharp: include dll.config file in .csproj

So as to find its contents when using (and searching with) an IDE.