macOS: Fix title not showing up in SelectFolderDialog, OpenFileDialog and SaveFileDialog (#3819)

macOS: Fix title not showing up in SelectFolderDialog, OpenFileDialog and SaveFileDialog

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

@@ -20,6 +20,7 @@ public:
             
             if(title != nullptr)
             {
+                panel.message = [NSString stringWithUTF8String:title];
                 panel.title = [NSString stringWithUTF8String:title];
             }
             
@@ -94,6 +95,7 @@ public:
             
             if(title != nullptr)
             {
+                panel.message = [NSString stringWithUTF8String:title];
                 panel.title = [NSString stringWithUTF8String:title];
             }
             
@@ -182,6 +184,7 @@ public:
             
             if(title != nullptr)
             {
+                panel.message = [NSString stringWithUTF8String:title];
                 panel.title = [NSString stringWithUTF8String:title];
             }