Fix tooltip display on Wayland

Hubert Figuière requested to merge hub/gnumeric:tooltip-fix into master

When typing a function the function tooltip would flicker and the console would have

Gdk-Message: 23:16:05.613: Window 0x2d08840 is a temporary window without parent, application will not be able to position it on screen.

On Wayland you must have a parent.

So in gnm_convert_to_tooltip() we call gtk_window_set_transient_for()

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