On macOS, set that we prefer tabbing

Winit now supports tabbing identifiers, thus set that we prefer tabbing,
in particular it'll make windows tab when using the same tabbing identifiers,
which is desirable for the end users.

src/platform_impl/macos/appkit/mod.rs

@@ -56,7 +56,8 @@ pub(crate) use self::version::NSAppKitVersion;
 pub(crate) use self::view::{NSTrackingRectTag, NSView};
 pub(crate) use self::window::{
     NSBackingStoreType, NSWindow, NSWindowButton, NSWindowLevel, NSWindowOcclusionState,
-    NSWindowOrderingMode, NSWindowSharingType, NSWindowStyleMask, NSWindowTitleVisibility,
+    NSWindowOrderingMode, NSWindowSharingType, NSWindowStyleMask, NSWindowTabbingMode,
+    NSWindowTitleVisibility,
 };
 
 #[link(name = "AppKit", kind = "framework")]

src/platform_impl/macos/appkit/window.rs

@@ -89,6 +89,9 @@ extern_methods!(
         #[sel(setSharingType:)]
         pub fn setSharingType(&self, sharingType: NSWindowSharingType);
 
+        #[sel(setTabbingMode:)]
+        pub fn setTabbingMode(&self, tabbingMode: NSWindowTabbingMode);
+
         #[sel(setOpaque:)]
         pub fn setOpaque(&self, opaque: bool);
 
@@ -425,3 +428,16 @@ pub enum NSWindowOrderingMode {
 unsafe impl Encode for NSWindowOrderingMode {
     const ENCODING: Encoding = NSInteger::ENCODING;
 }
+
+#[allow(dead_code)]
+#[repr(isize)] // NSInteger
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub enum NSWindowTabbingMode {
+    NSWindowTabbingModeAutomatic = 0,
+    NSWindowTabbingModeDisallowed = 2,
+    NSWindowTabbingModePreferred = 1,
+}
+
+unsafe impl Encode for NSWindowTabbingMode {
+    const ENCODING: Encoding = NSInteger::ENCODING;
+}

src/platform_impl/macos/window.rs

@@ -48,7 +48,7 @@ use super::appkit::{
     NSApp, NSAppKitVersion, NSAppearance, NSApplicationPresentationOptions, NSBackingStoreType,
     NSColor, NSCursor, NSFilenamesPboardType, NSRequestUserAttentionType, NSResponder, NSScreen,
     NSView, NSWindow, NSWindowButton, NSWindowLevel, NSWindowSharingType, NSWindowStyleMask,
-    NSWindowTitleVisibility,
+    NSWindowTabbingMode, NSWindowTitleVisibility,
 };
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
@@ -362,6 +362,7 @@ impl WinitWindow {
 
             if let Some(identifier) = pl_attrs.tabbing_identifier {
                 this.setTabbingIdentifier(&NSString::from_str(&identifier));
+                this.setTabbingMode(NSWindowTabbingMode::NSWindowTabbingModePreferred);
             }
 
             if attrs.content_protected {