CMake: build ImageJ as a gui application
CMake: build ImageJ as a gui application

By default, `add_executable` in CMake builds a console applicaiton,
which results in always popping a console when the application is
launched.

Adding the WIN32 flag builds the executable as a GUI app with no
automatic console.

imagej/imagej-launcher,imagej/imagej-launcher,imagej/imagej-launcher