Rework of the gstreamer importer.

To test on Windows:

1)  Install gstreamer 1.0 devel and runtime
2)  Add an GSTREAMER_SDK environment variable which points to the
    gstreamer base directory
3)  Uncomment USE_GSTREAMER in configwin.h

src/AudacityApp.cpp

@@ -71,7 +71,6 @@ It handles initialization and termination by subclassing wxApp.
 #include "effects/VST/VSTEffect.h"
 #include "widgets/ASlider.h"
 #include "FFmpeg.h"
-#include "GStreamerLoader.h"
 #include "Internat.h"
 #include "LangChoice.h"
 #include "Languages.h"
@@ -1264,10 +1263,6 @@ bool AudacityApp::OnInit()
    FFmpegStartup();
    #endif
 
-   #ifdef USE_GSTREAMER
-   GStreamerStartup();
-   #endif
-
    mImporter = new Importer;
 
    //