Add support for the color settings stack that XTerm copied from us without acknowledgement and decided to use incompatible escape codes for.

Completely in keeping with that project's past behavior.
See https://github.com/kovidgoyal/kitty/issues/879

XTerm announcement:
https://www.mail-archive.com/xorg@lists.x.org/msg06419.html

kitty_tests/screen.py

@@ -716,3 +716,31 @@ class TestScreen(BaseTest):
 
         self.ae(str(s.linebuf), '0\n5\n6\n7\n\n')
         self.ae(str(s.historybuf), '')
+
+    def test_color_stack(self):
+        s = self.create_screen()
+        c = s.callbacks
+
+        def w(code):
+            return parse_bytes(s, ('\033[' + code).encode('ascii'))
+
+        def ac(idx, count):
+            self.ae(c.wtcbuf, f'\033[{idx};{count}#Q'.encode('ascii'))
+            c.clear()
+
+        w('#R')
+        ac(0, 0)
+
+        w('#P')
+        w('#R')
+        ac(0, 1)
+        w('#10P')
+        w('#R')
+        ac(0, 1)
+        w('#Q')
+        w('#R')
+        ac(0, 0)
+        for i in range(20):
+            w('#P')
+        w('#R')
+        ac(9, 10)