decoder: Use GFile instead of PathBuf

This allows to show images from locations that don't
have a representation with path.

This also works around not being able to read files in trash:///

Closes #56

Part-of: <https://gitlab.gnome.org/Incubator/loupe/-/merge_requests/174>

src/widgets/sliding_view.rs

@@ -11,7 +11,6 @@ use indexmap::IndexMap;
 use once_cell::sync::{Lazy, OnceCell};
 
 use std::cell::{Cell, RefCell};
-//use std::path::{Path, PathBuf};
 
 static SCROLL_DAMPING_RATIO: f64 = 1.;
 static SCROLL_MASS: f64 = 0.5;