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/example_basic/CMakeLists.txt

@@ -17,13 +17,13 @@ include_directories(
 
 ### basic_plugin_example
 
-if(NOT CMAKE_SIZEOF_VOID_P MATCHES 4)
+if(NOT WORD_SIZE EQUAL 32)
 	if(WINDOWS)
 		add_definitions(/FIXED:NO)
 	else(WINDOWS) # not windows therefore gcc LINUX and DARWIN
 		add_definitions(-fPIC)
 	endif(WINDOWS)
-endif (NOT CMAKE_SIZEOF_VOID_P MATCHES 4)
+endif (NOT WORD_SIZE EQUAL 32)
 
 set(basic_plugin_example_SOURCE_FILES
     basic_plugin_example.cpp