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/llplugin/llpluginprocesschild.cpp

@@ -553,6 +553,15 @@ void LLPluginProcessChild::receivePluginMessage(const std::string &message)
 				}
 			}
 		}
+		else if (message_class == LLPLUGIN_MESSAGE_CLASS_INTERNAL)
+		{
+			std::string message_name = parsed.getName();
+			if(message_name == "shutdown")
+			{
+				// The plugin is finished.
+				setState(STATE_UNLOADING);
+			}
+		}
 	}
 	
 	if(passMessage)