Improve safety of haiku-drag-message

Improve safety of haiku-drag-message

* lisp/term/haiku-win.el (haiku-drag-and-drop): Ignore
placeholder message.
* src/frame.c (delete_frame): Prevent deleting drop source frame
on Haiku.
* src/haiku_support.cc (RELEASE_NOW, CANCEL_DROP): New message
types.
(class EmacsView, MessageReceived): Handle new message types.
(be_drag_message): Drag CANCEL_DROP message on quit; also send
RELEASE_NOW to view if quitting.

* src/haikuselect.c (syms_of_haikuselect)
(haiku_unwind_drag_message): Clear new frame variable.
(Fhaiku_drag_message): Set new frame variable.
* src/haikuterm.h: Update prototypes.