implement osx quit menu item.

native/Avalonia.Native/src/OSX/menu.mm

@@ -1,6 +1,7 @@
 
 #include "common.h"
 #include "menu.h"
+#include "window.h"
 
 @implementation AvnMenu
 @end
@@ -283,7 +284,8 @@ extern void SetAppMenu (NSString* appName, IAvnAppMenu* menu)
         auto quitItem = [[NSMenuItem alloc] init];
         quitItem.title = [@"Quit " stringByAppendingString:appName];
         quitItem.keyEquivalent = @"q";
-        quitItem.action = @selector(terminate:);
+        quitItem.target = [AvnWindow class];
+        quitItem.action = @selector(closeAll);
         [appMenu addItem:quitItem];
     }
     else

native/Avalonia.Native/src/OSX/window.h

@@ -15,6 +15,7 @@ class WindowBaseImpl;
 @end
 
 @interface AvnWindow : NSWindow <NSWindowDelegate>
++(void) closeAll;
 -(AvnWindow* _Nonnull) initWithParent: (WindowBaseImpl* _Nonnull) parent;
 -(void) setCanBecomeKeyAndMain;
 -(void) pollModalSession: (NSModalSession _Nonnull) session;

native/Avalonia.Native/src/OSX/window.mm

@@ -1076,6 +1076,16 @@ NSArray* AllLoopModes = [NSArray arrayWithObjects: NSDefaultRunLoopMode, NSEvent
     bool _isAppMenuApplied;
 }
 
++(void)closeAll
+{
+    auto numWindows = [[NSApp windows] count];
+    
+    for(int i = 0; i < numWindows; i++)
+    {
+        [[[NSApp windows] objectAtIndex:i] performClose:nil];
+    }
+}
+
 - (void)dealloc
 {
 }