Add AIDirPicker and use it. Remove indra/newview/ll{dir,file}picker.{h,cpp}.

Also removed some code from the Mac/windows code in
indra/plugins/filepicker/llfilepicker.cpp that shouldn't
be in there anymore (send_agent_pause/resume and updating
the LLFrameTimer stuff).

indra/newview/llviewermessage.cpp

@@ -49,7 +49,6 @@
 #include "llchat.h"
 #include "lldbstrings.h"
 #include "lleconomy.h"
-#include "llfilepicker.h"
 #include "llfocusmgr.h"
 #include "llfollowcamparams.h"
 #include "llinstantmessage.h"