Switch to higher-level DTMF API

We can get a DTMF sender from the session or even the chat, rather than
manually wrapping the track.

Handle ^D / EOT since we asked the terminal not to

Don't use atexit anymore

It turns out we only run the cleanup code in a few of the exit paths, so
let's do it explicitly.

Support sending DTMF from stdin

Fix Makefile

README

AppImage build

First commit