Add FLTK color theming support (PR #139 by @Villardo)
It's now possible to select one of the FLTK color themes (Shake, Tan, Gray, Dark, Black) for the application.
Windows setup and release/debug binaries in assets.
Linux appimages for Debian Bookworm and derivatives (LMDE6, libc 2.36), and Ubuntu 20.04LTS (Linux Mint 20, libc 2.31) in assets.