Split plugin classes and derive AIFilePicker from BasicPluginBase (part 4).

Add back fixes that were in Singularity (in indra/media_plugins) but not
in imprudence.

Also:

Add "shutdown" plugin message and terminate file picker plugins cleanly.

The DSO (libbasic_plugin_filepicker.so) now tells the child process /
plugin loader (SLPlugin) to terminate (using the 'shutdown' message),
and AIFilePicker::finish_impl destroys the plugin object on the viewer
side when it's ... finished thus.

Also added a large comment that gives an overview of all classes
involved on the viewer side.

Additional fixes for filepicker.

Plugin refactor bug fix: mDeleteMe was uninitialized in AIPluginFilePicker.

indra/plugins/gstreamer010/media_plugin_gstreamer010.cpp

@@ -652,7 +652,7 @@ bool
 MediaPluginGStreamer010::getTimePos(double &sec_out)
 {
 	bool got_position = false;
-	if (mPlaybin)
+	if (mDoneInit && mPlaybin)
 	{
 		gint64 pos;
 		GstFormat timefmt = GST_FORMAT_TIME;