This is just a skeleton, roughly copied from the Postcoard
code, which is very similar. Nothing in indra/newview/llfloaterfeed.cpp
has been changed yet, apart from a few deletions (to make it compile)
and the literal replacement of "Postcard" with "Feed".
indra/newview/llfloatersnapshot.cpp was updated however to
open the new floater when one tries to upload a snapshot to the feed.
It is still needed to make a callback to saveFeed(), or rather to
move that function to the saveFeed that is in llfloatersnapshot.cpp.
aab978c7ea