When pressing return in `nano` the terminal received an `<CTRL>+J` instead of
the return key. `<CTRL>+J` justifies the text. But is (confusingly) actually
the key for a line feed as well? Terminal programming is not easy …
* demo app: using `SHELL` environment var or `/bin/bash` as default
* removed some debug messages
* enabled all vterm screen handlers; if not implemented there is a debug
message
* fixed and improved cursor position — was sometimes off; e. g. if
pressing backspace
* stubs for some functions required for implementing #3