gtkwindow: apply csd offset to set/get_default_size
gtkwindow: apply csd offset to set/get_default_size

An application may use gtk_window_get_size() to retrieve the current
window size and later reuse that size with
gtk_window_set_default_size().

gtk_window_set_default_size() and gtk_window_get_default_size() should
also take client side decorations offset into account.

Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=756618

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