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. It could appeal
to prospective users who refrain from trying stb-tester because of the
command line "wizardy" it requires.

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

"config" menu item is different from `stbt config`: it brings up a
simple text 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.

Dialog boxes are created with Zenity. It's pre-installed with Gnome
and available in MacPorts.
stbt launcher: Add applications menu item

Create an applications menu item for `stbt launcher` on Linux when
installing the RPM package or by calling `make menuitem`.

The motivation behind this is that `stbt launcher` is targeted for
users who prefer not to use the command line.

The icon is a copy of stb-tester-350px.svg from the stb-tester.com.git
repository, resized to 256x256px.