Rename kitty-tool to kitten

It's easier to type, and cuter.

Also, most, if not all of the TUI parts of
kitty's kittens will eventually be re-written into kitten.

The only downside I can see is that we cant tab complete kitty
anymore, but hopefully there will be less reason to run kitty
from the shell as command line tools migrate to kitten.

Meowrrrr!!!

kitty/tabs.py

@@ -28,7 +28,7 @@ from typing import (
 from .borders import Border, Borders
 from .child import Child
 from .cli_stub import CLIOptions
-from .constants import appname, kitty_tool_exe
+from .constants import appname, kitten_exe
 from .fast_data_types import (
     GLFW_MOUSE_BUTTON_LEFT,
     GLFW_MOUSE_BUTTON_MIDDLE,
@@ -441,7 +441,7 @@ class Tab:  # {{{
                                     cmd[:0] = shlex.split(line)
                                 else:
                                     cmd[:0] = [resolved_shell(get_options())[0]]
-                                cmd[:0] = [kitty_tool_exe(), '__hold_till_enter__']
+                                cmd[:0] = [kitten_exe(), '__hold_till_enter__']
         fenv: Dict[str, str] = {}
         if env:
             fenv.update(env)