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/base_basic/basic_plugin_base.h

@@ -57,6 +57,9 @@ public:
 	// This function is actually called and then calls the member function above.
 	static void staticReceiveMessage(char const* message_string, BasicPluginBase** self_ptr);
 
+	// Shoot down the whole process.
+	void sendShutdownMessage(void);
+
 protected:
 	void sendMessage(LLPluginMessage const& message);