Tests show that on Windows, for a window to indicate it will accept a
file drop or to actually accept it, it must have WS_EX_ACCEPTFILES in
its style, contain DROPEFFECT_COPY (which will be chosen if offered
among several drop effects), and be offered the HDROP format.

Windows doesn't send WM_DROPFILES to a window if a drop target is
registered on that window. Some apps that support both even duplicate
dropped files if we send it and a drop target is registered.

Also fixes a bug in UnDecorateSymbolName when undecorated_length == 0.