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/import/ImportGStreamer.h

@@ -13,9 +13,6 @@ LRN
 
 #include "ImportPlugin.h"
 
-class ImportPluginList;
-class UnusableImportPluginList;
-
 void GetGStreamerImportPlugin(ImportPluginList *importPluginList,
                            UnusableImportPluginList *unusableImportPluginList);