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/newview/llmediactrl.h

@@ -34,7 +34,7 @@
 #define LL_LLMediaCtrl_H
 
 #include "llviewermedia.h"
-
+#include "llviewermediaobserver.h"
 #include "lluictrl.h"
 #include "llframetimer.h"
 #include "lldynamictexture.h"