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/filepicker/basic_plugin_filepicker.cpp

@@ -150,6 +150,10 @@ void FilepickerPlugin::receiveMessage(char const* message_string)
 				message.setValue("plugin_version", plugin_version);
 				sendMessage(message);
 			}
+			else if (message_name == "cleanup")
+			{
+				// We have no resources that need care. Just do nothing.
+			}
 			else if (message_name == "idle")
 			{
 				// This whole message should not have existed imho -- Aleric
@@ -213,6 +217,8 @@ void FilepickerPlugin::receiveMessage(char const* message_string)
 					message.setValueLLSD("filenames", filenames);
 					sendMessage(message);
 				}
+				// We're done. Exit the whole application.
+				sendShutdownMessage();
 			}
 			else
 			{