stbt remote: New command line tool to send RCU commands with PC keyboard

This is useful when multiple set-top-boxes are under test in the same
room and it is not safe to use the remote controller shipped with the
box to control it manually because the IR signals are received by multiple
boxes.

The tool uses the same remote control implementations as 'stbt run'.

A keymap file is required to save mappings of keyboard keys to remote
control keys. It was designed to make it easy to create a keymap from
scratch manually. See `stbt remote --help` for detailed information.