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/window.rs

@@ -678,9 +678,9 @@ impl LpWindow {
     // have multiple `match` or `if let` branches, and without needing
     // to unwrap.
     #[template_callback]
-    fn window_title(&self, path: glib::variant::Variant) -> String {
+    fn window_title(&self, file: Option<&gio::File>) -> String {
         // ensure that templates are initialized
-        if path.as_maybe().is_none() {
+        if file.is_none() {
             gettext("Loupe")
         } else {
             self.imp()