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/llpluginprocessparent.h

@@ -53,6 +53,7 @@ public:
 	virtual ~LLPluginProcessParentOwner();
 	virtual void receivePluginMessage(const LLPluginMessage &message) = 0;
 	virtual bool receivePluginMessageEarly(const LLPluginMessage &message) {return false;};
+	virtual void receivedShutdown() = 0;
 	// This will only be called when the plugin has died unexpectedly 
 	virtual void pluginLaunchFailed() {};
 	virtual void pluginDied() {};
@@ -88,6 +89,9 @@ public:
 	// Go to the proper error state
 	void errorState(void);
 
+	// Go to exit state.
+	void exitState(void) { setState(STATE_EXITING); }
+
 	void setSleepTime(F64 sleep_time, bool force_send = false);
 	F64 getSleepTime(void) const { return mSleepTime; };