general: Use std's OnceCell where possible

Rust's standard library now provides a drop-in replacement
for `OnceCell`. It requires no API changes from `once_cell`'s
version.

This commit replaces the version from `once_cell` with the
new `std` struct where possible. We still need to use
`once_cell` for properties until
https://github.com/gtk-rs/gtk-rs-core/pull/1113
is merged. We also still need it for Lazy until that lands.

Part-of: <https://gitlab.gnome.org/GNOME/Incubator/loupe/-/merge_requests/241>

Cargo.toml

@@ -2,7 +2,7 @@
 name = "loupe"
 version = "0.1.0"
 edition = "2021"
-rust-version = "1.65"
+rust-version = "1.70"
 license = "GPL-3.0-or-later"
 
 [profile.release]

src/file_model.rs

@@ -26,8 +26,8 @@ use gio::prelude::*;
 use gio::subclass::prelude::*;
 use glib::GString;
 use indexmap::IndexMap;
-use once_cell::sync::{Lazy, OnceCell};
+use once_cell::sync::Lazy;
-use std::cell::RefCell;
+use std::cell::{OnceCell, RefCell};
 
 mod imp {
     use super::*;

src/widgets/image.rs

@@ -41,9 +41,8 @@ use adw::{prelude::*, subclass::prelude::*};
 use arc_swap::ArcSwap;
 use futures::prelude::*;
 use once_cell::sync::Lazy;
-use once_cell::unsync::OnceCell;
 
-use std::cell::{Cell, RefCell};
+use std::cell::{Cell, OnceCell, RefCell};
 use std::sync::Arc;
 
 /// Default background color around images and behind transparent images

src/widgets/image_view.rs

@@ -54,7 +54,7 @@ use std::cell::{Cell, RefCell};
 const BUFFER: usize = 2;
 
 mod imp {
-    use once_cell::sync::OnceCell;
+    use std::cell::OnceCell;
 
     use super::*;
 

src/widgets/sliding_view.rs

@@ -27,9 +27,9 @@ use crate::widgets::LpImagePage;
 use adw::prelude::*;
 use adw::subclass::prelude::*;
 use indexmap::IndexMap;
-use once_cell::sync::{Lazy, OnceCell};
+use once_cell::sync::Lazy;
 
-use std::cell::{Cell, RefCell};
+use std::cell::{Cell, OnceCell, RefCell};
 
 static SCROLL_DAMPING_RATIO: f64 = 1.;
 static SCROLL_MASS: f64 = 0.5;

src/window.rs

@@ -30,9 +30,7 @@ use adw::subclass::prelude::*;
 use glib::clone;
 use gtk::CompositeTemplate;
 
-use once_cell::sync::OnceCell;
+use std::cell::{Cell, OnceCell, RefCell};
-
-use std::cell::{Cell, RefCell};
 use std::path::{Path, PathBuf};
 
 use crate::config;