CHOP-763: Move llwindowlistener.{h,cpp} from llwindow to newview.

Instantiate LLWindowListener on LLViewerWindow instead of on LLWindow.
This permits LLWindowListener to use machinery from llui, e.g.
LLUI::resolvePath().
Document planned new ["path"], ["reply"] params to "keyDown", "keyUp",
"mouseDown", "mouseUp", "mouseMove" operations; document relationship between
["path"] and ["x"] and ["y"].
NEW PARAMS NOT YET IMPLEMENTED.