Feature message for Qt::Widgets, and a #define in config-gammaray.h.
QCoreApplication is enough here.
Make injectors build without widgets.
Split out shared code needed for the CLI and UI launcher into a static lib.
Split thelauncher into an UI part and a CLI part.

This allows us to only build the CLI part if we do not have QtWidgets
available on a target, so GammaRay is usable there, although without
the UI convenience.
Factor out duplicated code to find helper executables.