Split plugin classes and derive AIFilePicker from BasicPluginBase (part 1).

This commit contains all changes, except those in indra/media_plugins
(which was renamed to indra/plugins) and indra/llplugin.

However, it does contain the (new) file
indra/plugins/filepicker/basic_plugin_filepicker.cpp

indra/CMakeLists.txt

@@ -76,8 +76,8 @@ if (VIEWER)
   add_subdirectory(${LIBS_OPEN_PREFIX}llplugin)
   add_subdirectory(${LIBS_OPEN_PREFIX}llui)
 
-  # viewer media plugins
-  add_subdirectory(${LIBS_OPEN_PREFIX}media_plugins)
+  # viewer plugins directory
+  add_subdirectory(${LIBS_OPEN_PREFIX}plugins)
 
   # llplugin testbed code (is this the right way to include it?)
   #if (NOT LINUX)