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/llviewerparcelmedia.h

@@ -34,6 +34,7 @@
 #define LLVIEWERPARCELMEDIA_H
 
 #include "llviewermedia.h"
+#include "llviewermediaobserver.h"
 
 // For use by other patches so they know that media filtering is implemented.
 #define MEDIA_FILTERING 1
@@ -42,7 +43,6 @@ class LLMessageSystem;
 class LLParcel;
 class LLViewerParcelMediaNavigationObserver;
 
-
 // This class understands land parcels, network traffic, LSL media
 // transport commands, and talks to the LLViewerMedia class to actually
 // do playback.  It allows us to remove code from LLViewerParcelMgr.