gio: icons should fallback to non-preferred style appropriately.
Whatever the preferred icon style is (symbolic, regular or the requested style), fallbacking to the other style in case of absent variant is better than not finding any icon at all. Also style fallbacking should be managed separately from property "use-default-fallbacks". Default fallbacks are meant for the process of getting up in context levels (as separated by dashes in icon name). Even though it also uses dash characters in format, this is a different concept as the variant of styles. Without this commit for instance, if an icon only had a symbolic variant, and the theme had "-gtk-icon-style" set to "regular" while your GTK+ application requested the regular icon name, you were getting no icons, and the application would look completely broken. Now one would at least fallback to the symbolic icon as last resort (which is infinitely better than having no icons).
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