[WinForms] Fix NotifyIcon not showing in systray on 64bit linux (#16164)
https://bugzilla.xamarin.com/show_bug.cgi?id=14976
Credit goes to datnhanlz for noting that removing XChangeProperty(DisplayHandle, hwnd.whole_window, _XEMBED_INFO, _XEMBED_INFO, 32, PropertyMode.Replace, atoms, 2); would also fix the bug.
Note that the type of Atom is actually unsigned long (see usr/include/X11/X.h), however the code used a C# int for the atoms. As such the code worked without a problem on 32 bit linux, but broke on 64 bit linux.
I switched to using IntPtr instead to make the code also work on 64-bit.