Implement `allow-same-frame' for NS drag-and-drop

Implement `allow-same-frame' for NS drag-and-drop

* lisp/term/ns-win.el (x-begin-drag): Implement
`allow-same-frame'.
* src/nsselect.m (Fns_begin_drag): New parameter
`allow-same-frame'.
* src/nsterm.h (@interface EmacsWindow): Update prototypes.
* src/nsterm.m ([EmacsView draggingEntered:]):
([EmacsView prepareForDragOperation:]):
([EmacsView performDragOperation:]): Respect new parameter.
([EmacsWindow beginDrag:forPasteboard:withMode:returnFrameTo:]):
Likewise.