Move iOS and macOS implementations into new apple module (#3756)

Move iOS and macOS implementations to a shared folder called `apple`, to allow
us to reduce the code-duplication between these platforms in the future.

The folder structure is now:
- `src/platform_impl/apple/`
  - `appkit/`
  - `uikit/`
  - `example_shared_file.rs`
  - `mod.rs`

* Add preliminary support for tvOS, watchOS and visionOS
* Reduce duplication in Cargo.toml when specifying dependencies

src/platform_impl/ios/mod.rs

@@ -1,52 +0,0 @@
-#![allow(clippy::let_unit_value)]
-
-mod app_delegate;
-mod app_state;
-mod event_loop;
-mod monitor;
-mod view;
-mod view_controller;
-mod window;
-
-use std::fmt;
-
-use crate::event::DeviceId as RootDeviceId;
-
-pub(crate) use self::event_loop::{
-    ActiveEventLoop, EventLoop, EventLoopProxy, OwnedDisplayHandle,
-    PlatformSpecificEventLoopAttributes,
-};
-pub(crate) use self::monitor::{MonitorHandle, VideoModeHandle};
-pub(crate) use self::window::{PlatformSpecificWindowAttributes, Window, WindowId};
-pub(crate) use crate::cursor::{
-    NoCustomCursor as PlatformCustomCursor, NoCustomCursor as PlatformCustomCursorSource,
-};
-pub(crate) use crate::icon::NoIcon as PlatformIcon;
-pub(crate) use crate::platform_impl::Fullscreen;
-
-/// There is no way to detect which device that performed a certain event in
-/// UIKit (i.e. you can't differentiate between different external keyboards,
-/// or whether it was the main touchscreen, assistive technologies, or some
-/// other pointer device that caused a touch event).
-#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub struct DeviceId;
-
-impl DeviceId {
-    pub const unsafe fn dummy() -> Self {
-        DeviceId
-    }
-}
-
-pub(crate) const DEVICE_ID: RootDeviceId = RootDeviceId(DeviceId);
-
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
-pub struct KeyEventExtra {}
-
-#[derive(Debug)]
-pub enum OsError {}
-
-impl fmt::Display for OsError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "os error")
-    }
-}