Add a wxGTK-specific function to set wxNotificationMessage icon name.

Add a wxGTK-specific function to set wxNotificationMessage icon name.

It's trivial to use a stock icon with the given name in wxGTK with libnotify,
so provide a way to do it. However this is not as simple as that in other
implementations (notably Windows), so make it private to this port for now.

In the future we should try to support arbitrary wxIcons as well as extend
wxIconLocation to support FreeDesktop stock icon names.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72250 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775