Panel dialogs are inconsistent with use of icons

This was reported on the elementary-xfce icon theme, but it is an issue with xfce-panel so I'm just reporting/documenting it here.

Some panel dialogs use libxfce4ui, like Panel Preferences > Remove Panel (code here)

Others don't, like when you right click on a plugin in and select Remove (code here)

Or when you use the Action Buttons > Shutdown (or any other action that countdown confirms) (code here)

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I took a look at it but couldn't get a working fix. I might try again at some point in the future, but maybe someone else can do a better job with it.