GdkSurface - Don't call unimplemented NULL set_startup_id() vfunc on Win32

Merged Sebastian Dröge requested to merge sdroege/gtk:gdk-win32-unimplement-vfuncs-3.24 into gtk-3-24

GdkSurface::set_startup_id() is NULL on Win32 and would cause a segfault if called.

While the documentation of the main caller of set_startup_id(), gtk_window_set_startup_id(), mentions that it's not implemented on Windows it can still be automatically called via Glade and simply doing nothing on Win32 is going to be less disruptive than a segfault.

See and !825 (merged) for the master MR.

Edited by Sebastian Dröge

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