The mailwatch plugin needs some icons provided by the xfce4-icon-theme Rodent.
However, if you use another icon theme, the icons are not found and so they are
not displayed.

However, other xfce apps (like the panel) install default icons in
/usr/share/icons/hicolor that serve als fallback if you choose another icon
theme. So they maybe don't fit into the theme, but they are here.

It would be good if the mailwatch plugin could do the same. For now, I just
copied them.

Reproducible: Always
Steps to Reproduce:
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