stbt control: Use as control recorder with stbt record

A new listener type that integrates `stbt control` with `stbt record`.

Launching `stbt record` as `stbt record --control-recorder=stbt-control`
brings up `stbt control` to record keypresses using the PC keyboard.

The control URI is 'stbt-control:<keymap_file>' where <keymap_file> is
optional and defaults to ~/.config/stbt/control.conf.

stbt.py imports stbt-control to call its main loop, which generates
the bytecode cache 'stbt-controlc'; as stbt-control doesn't have '.py'
extension, it is not '.pyc' but gets the 'c' ending. This file is
cleaned up similarly to '.pyc' files.