Add a new escape code to allow terminal programs to trigger desktop notifications

Fixes #1474

kitty_tests/__init__.py

@@ -37,10 +37,14 @@ class Callbacks:
     def use_utf8(self, on):
         self.iutf8 = on
 
+    def desktop_notify(self, osc_code: int, raw_data: str) -> None:
+        self.notifications.append((osc_code, raw_data))
+
     def clear(self):
         self.wtcbuf = b''
         self.iconbuf = self.titlebuf = self.colorbuf = self.ctbuf = ''
         self.iutf8 = True
+        self.notifications = []
 
 
 def filled_line_buf(ynum=5, xnum=5, cursor=Cursor()):