change white background for icons in PouInstanceVariablePanel to transparent

in wxPython 2.8 panel has white background and therefore white
background is appropriate, but in wxPython 3.0 panel has grey
background (on GNU/Linux and on Windows).
Transparent background looks good on both wxPython versions and on all platforms.

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<?xml version="1.0" encoding="UTF-8"?>

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<CanFestivalInstance/>