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/webkit/linux_volume_catcher.cpp

@@ -42,9 +42,8 @@
   5) Keep a list of all living audio players that we care about, adjust the volumes of all of them when we get a new setVolume() call
  */
 
-# include <set> //imprudence
-
 #include "linden_common.h"
+# include <set>
 
 #include "volume_catcher.h"