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/CMakeLists.txt

@@ -51,9 +51,9 @@ set(media_plugin_webkit_LINK_LIBRARIES
 
 # Select which VolumeCatcher implementation to use
 if (LINUX)
-  if (PULSEAUDIO)
+  if (PULSEAUDIO_FOUND)
     list(APPEND media_plugin_webkit_SOURCE_FILES linux_volume_catcher.cpp)
-  endif (PULSEAUDIO)
+  endif (PULSEAUDIO_FOUND)
   list(APPEND media_plugin_webkit_LINK_LIBRARIES
        ${UI_LIBRARIES}     # for glib/GTK
        )