API: gdk: Change get_drag_window() API

The previous function gdk_drag_get_protocol_for_display() took native
window handles, so it had to be changed. Because it didn't do what it
was named to do (it didn't return a protocol even though it was named
get_protocol) and because it doesn't operate on the display anymore but
on the actual window, it's now called gdk_window_get_drag_protocol().

docs/reference/gdk/gdk3-sections.txt

@@ -882,11 +882,11 @@ gdk_drag_begin
 gdk_drag_begin_for_device
 gdk_drag_motion
 gdk_drop_finish
-gdk_drag_get_protocol_for_display
 GdkDragProtocol
 GdkDragAction
 gdk_drag_status
 gdk_drag_drop_succeeded
+gdk_window_get_drag_protocol
 
 gdk_drag_context_get_actions
 gdk_drag_context_get_suggested_action