File Browser: Change from "nfd2" to "rfd" for showing the file dialogs

Apparently "rfd" is the new standard crate for this stuff,
as it's 100% Rust, whereas nfd2 still required a C-compiler
to use. As such, "nfd2" had apparently been abandoned back
in August 2021, though this wasn't clear from the crates.io
or other sources when I checked yesterday. So, this app should
now be 100% Rust again now.

The other main advantage is that now the file dialog only shows
file types we can load by default.