Stop using &mut in Objective-C delegate methods (#2925)

* Make iOS declared classes not use &mut

* Prepare `init` methods for not having access to &mut self

* Prepare WinitWindow methods for not having access to &mut self

* Convert a bit of WinitView's to use interior mutability

* Convert a bit more of WinitView's to use interior mutability

* Convert the rest of WinitView to use interior mutability

* Use interior mutability instead of a Mutex for the CursorState

* Use interior mutability in WinitWindowDelegate

src/platform_impl/ios/ffi.rs

@@ -70,6 +70,10 @@ impl From<UIUserInterfaceIdiom> for Idiom {
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct UIRectEdge(NSUInteger);
 
+impl UIRectEdge {
+    pub(crate) const NONE: Self = Self(0);
+}
+
 unsafe impl Encode for UIRectEdge {
     const ENCODING: Encoding = NSUInteger::ENCODING;
 }