nwg-menu (MenuStart for sway and other wlroots-based compositors)

This code provides the MenuStart plugin to nwg-panel. It also may be
used standalone, however, with a little help from command line
arguments. This program is being developed with sway in mind. It
should work with other wlroots-based Wayland compositors, but for
now it's only been tested briefly on Wayfire.
