Implement a slowrx daemon

[sjlongland]

This pull request separates the SSTV decoder logic from the UI a little bit, allowing a separate command-line only SSTV daemon to be implemented.

This daemon could be started via means such as using systemd to make a headless SSTV receiver, optionally running a script that can perform actions on the received images.

• The core library now uses standard integer and booleans (stdint.h and stdbool.h)
• Rather than directly calling GTK routines, the core and UI communicate via shared global variables and callback functions (function pointers).
• FFTW3's fftw_complex becomes a typedef for double complex if complex.h is imported, so that has been done to further reduce the non-standard types.
• The daemon uses libgd to write the image file, simply because this is more likely to be available on a headless server than a GUI library like GDK.
• The GUI functions as it does before and uses the same libraries for rendering output images.

The code is a little rough, but seems to be working.