please implement an FLTK-based backend

[pdhahn]

I am new to polyscope and per my initial evaluation, it looks really great for use as a 3d visualization tool in my projects.

However, I am heavily invested in using the GUI toolkit FLTK for UI's in my current code.

Could you please implement an FLTK backend in addition to the GLFW + ImGUI one you already provide? It already provides a wrapper class for opengl windows, called FL_Gl_Window. Moreover, it's rich set of GUI widgets (buttons, slider, menus, etc.) can be used anywhere on that window (meaning ImGUI is not needed).

The released version of FLTK I use is 1.4 although I also use the "bleeding edge" version 1.5, so either works fine for me. Both are available as source repos on github.

Thank you for considering this!

Paul