If isatty(STDOUT_FILE) is true, then we can possibly send
escape sequences to set the title of the terminal. This is
useful for both terminals like gnome-terminal as well as
programs scripting flatpak-builder such as Builder. This allows
those programs to get information about the current build
progress without having to parse the command line output which
is muxed with build information.
A new user might think that APPDIR is the location of the app to be
built, and run something like `xdg-app-builder . ./manifest`. This
could silently the user's entire project that they are trying to
package, which is not acceptable at all! Even if you think it is their
fault for not reading the manual first!
This commit means that APPDIR is no longer deleted. Instead,
xdg-app-builder checks whether it is empty and, if it is not, it asks
the user to delete the contents and then rerun it.
This means you now have to do `rm -Rf APPDIR; xdg-app-builder APPDIR
MANIFEST` when developing your manifest, but I think that's better than
having a build tool that can optionally delete your whole project.
