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' and
'stbt record'.

A keymap file is required to save mappings of keyboard keys to remote
control keys. The keymap file can be recorded using the tool's
'recording mode' but it was designed to make it easy to create one from
scratch manually. See `stbt remote --help` for detailed information.

As 'curses' library waits for 2 seconds for further keycodes after it
receives an 'Escape' character, using 'Escape' is quite inconvenient and
therefore is not allowed in this implementation.
stbt remote: Fix a false negative Pylint error

Pylint 0.25 reports the error "Instance of 'list' has no 'strip'
member" for 'subprocess.check_output(...).strip()', which is a
false negative because 'check_output' returns a newline-separated
string.

Casting the return value to 'str' resolves this issue.