icons: fix symbolic icons.
With the symbolic-awareness of GTK+, vector icons are recolored (using fg, success, warning and error colors). Unfortunately it was also recoloring some rectangles with neither fill nor stroke, which were using only to get appropriate icon size (these icons were therefore displayed as just a square). I made a quick pass in our icons to "fix" these. I expect I may have missed some icons, or at the opposite, I may have broken other icons by removing rectangles which should not have been removed. If you find any such icon, please fix. Also some icons do not look quite right just now. We will have to create new guidelines on how to make symbolic icons and fix/redesign some, I guess. (cherry picked from commit 0c3659c5)
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