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.
c46c86ca4b