GTK dependency?

[nico-abram]

I got a build error saying cargo:warning=nativefiledialog/src/nfd_gtk.c:10:21: fatal error: gtk/gtk.h: No such file or directory when trying to run a binary using this in travis CI. It would be nice if the dependency was specified in the readme.

[selassje]

I tried installing libgtk2.0-dev but the problem still persists with travis