launcher: GUI wrapper for stb-tester commands

A simple GUI to launch stbt terminal commands, targeted for users who don't feel comfortable with the command line.

The tool uses Zenity to create dialog windows. In theory Zenity should work with OS X as it's available via MacPorts; in practice I haven't been able to make it work.

Added as a standalone tool in `extra`. My original idea was to make it a stbt tool (available as `stbt launcher`) so an applications menu item can be added, as the tool is for users who prefer not to use the command line. However, as the tool is not cross-platform and targeted to a small subset of stbt users, it's better suited as an `extra` script.