Refactor the drop protocol

Get rid of request_id. Instead use the x, y, and Y fields to
disambiguate requests. Specify error handling a little more robustly.

Implementation needed.

gen/apc_parsers.py

@@ -333,11 +333,10 @@ def parsers() -> None:
     write_header(text, 'kitty/parse-multicell-command.h')
 
     keymap = {
-        't': ('type', flag('aAmMrRsdoOpPeE')),
+        't': ('type', flag('aAmMrRoOpPeE')),
         'm': ('more', 'uint'),
         'i': ('client_id', 'uint'),
         'o': ('operation', 'uint'),
-        'r': ('request_id', 'uint'),
         'x': ('cell_x', 'int'),
         'y': ('cell_y', 'int'),
         'X': ('pixel_x', 'int'),