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/llpluginclassbasic.h

@@ -113,6 +113,9 @@ protected:
 	// Inherited from LLPluginProcessParentOwner.
 	/*virtual*/ void receivePluginMessage(LLPluginMessage const&);
 
+	// Inherited from LLPluginProcessParentOwner.
+	/*virtual*/ void receivedShutdown() { mPlugin->exitState(); }
+
 	//--------------------------------------
 	// Debug use only
 	//