Implement some drag and drop functions on NS

Implement some drag and drop functions on NS

* lisp/term/ns-win.el (ns-selection-exists-p):
(gui-backend-set-selection):
(x-begin-drag): New functions and selection types.
* src/nsfns.m (Fns_get_resource):
(Fns_set_resource):
(Fx_server_max_request_size): Fix coding style.
* src/nsselect.m (ns_decode_data_to_pasteboard):
(ns_lisp_to_pasteboard):
(ns_dnd_action_to_operation):
(ns_dnd_action_from_operation):
(Fns_begin_drag): New functions.
(syms_of_nsselect): New subrs.
* src/nsterm.h (EmacsWindow): New fields and messages.
(NSPasteboardNameGeneral): New define.
* src/nsterm.m ([EmacsView mouseDown:]): Store last mouse event.
([EmacsWindow initWithEmacsFrame:fullscreen:screen:]): Clear that event.
([EmacsWindow dealloc]): Free last mouse event.