Fix non-usable toolbar on wxPython with GTK3+

On GNU/Linux if wxPython uses wxWidget with GTK3+ backend (this is
what most modern distributions do), then size of EditorToolBar was
always to store only one button.
This is because GetBestSize() is updated only after pane is shown and
updated.
The code does work correctly on python-wxgtk3.0 (gtk2 backend) on
GNU/Linux and with wxpython 2.8 on Windows.

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Trace POUs instances variable

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