Fix nightly warnings

eg:
```
warning: the item `Box` is imported redundantly
```

Author: ogoffart

api/cpp/cbindgen.rs

@@ -3,7 +3,6 @@
 
 use anyhow::Context;
 use std::io::{BufWriter, Write};
-use std::iter::Extend;
 use std::path::{Path, PathBuf};
 
 // cSpell: ignore compat constexpr corelib deps sharedvector pathdata

api/cpp/platform.rs

@@ -1,9 +1,9 @@
 // Copyright © SixtyFPS GmbH <[email protected]>
 // SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-1.1 OR LicenseRef-Slint-commercial
 
-use alloc::boxed::Box;
 use alloc::rc::Rc;
-use alloc::string::String;
+#[cfg(not(feature = "std"))]
+use alloc::{boxed::Box, string::String};
 use core::ffi::c_void;
 use i_slint_core::api::{
     LogicalSize, PhysicalPosition, PhysicalSize, Window, WindowPosition, WindowSize,

internal/backends/qt/qt_accessible.rs

@@ -16,7 +16,6 @@ use cpp::*;
 use pin_project::pin_project;
 use qttypes::QString;
 
-use alloc::boxed::Box;
 use core::ffi::c_void;
 use std::pin::Pin;
 

internal/core/graphics/image.rs

@@ -1007,7 +1007,6 @@ pub fn fit9slice(
 pub(crate) mod ffi {
     #![allow(unsafe_code)]
 
-    use super::super::IntSize;
     use super::*;
 
     // Expand Rgb8Pixel so that cbindgen can see it. (is in fact rgb::RGB<u8>)

internal/core/item_tree.rs

@@ -986,7 +986,6 @@ pub(crate) mod ffi {
     #![allow(unsafe_code)]
 
     use super::*;
-    use crate::slice::Slice;
     use core::ffi::c_void;
 
     /// Call init() on the ItemVTable of each item in the item array.

internal/core/properties/ffi.rs

@@ -4,7 +4,6 @@
 use super::*;
 use crate::graphics::{Brush, Color};
 use crate::items::PropertyAnimation;
-use core::pin::Pin;
 
 #[allow(non_camel_case_types)]
 type c_void = ();