stbt launcher: GUI wrapper for stb-tester commands

A simple GUI to launch stbt terminal commands. Users of stb-tester who
are not used to the command line should find it helpful.

Mandatory command line arguments are specified through dialog windows;
optional parameters are read from the stbt config file, or default
values are used.

"config" menu item is different from `stbt config`: it brings up a
simple file editor to modify the config file. ~/.config/stbt/stbt.conf
is opened by default, or the value of $STBT_CONFIG_FILE if it's in the
environment. If the default config file doesn't exist, the tool creates
it for the user.