Directly invoke platform-specific TTS commands.

Pyttsx would try to invoke TTS from the python process, which
required a ton of dependencies. (Pyobjc and win32api) We now
use subprocess to run platform-specific executables, passing
the text to speak on the commands line.
tts: Make nvl-mode compatible with tts.

This involves using alt to disable speaking of already-spoken text.
tts: Read non-prefs labels.

We want to read yesno_label, for example.
tts: Add self-voicing text to file actions.
tts: Add a new linear focus ordering.

This allows one to navigate through all of the displayables on the
screen with the cursor keys, without having to do 2-d navigation,
which is what Ren'Py does by default.
tts: Allow the user to unfocus all widgets.

This will read out the default text.
tts: Speak bar text immediately.