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>

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::*;