Implement OS.request_attention() for OSX (#5662)

Keeps bouncing icon until user focuses window

platform/osx/os_osx.h

@@ -202,7 +202,7 @@ public:
 	virtual bool is_window_minimized() const;
 	virtual void set_window_maximized(bool p_enabled);
 	virtual bool is_window_maximized() const;
-
+	virtual void request_attention();
 
 	void run();
 

platform/osx/os_osx.mm

@@ -1577,6 +1577,11 @@ void OS_OSX::move_window_to_foreground() {
 	[window_object orderFrontRegardless];
 }
 
+void OS_OSX::request_attention() {
+
+	[NSApp requestUserAttention:NSCriticalRequest];
+}
+
 String OS_OSX::get_executable_path() const {
 
 	int ret;