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).

For the record, we need this in GIMP (in our master branch which is GTK+3). Right now, without this, the icon theming experience is very broken IMO. Also, checking carefully the CSS properties, the -gtk-icon-style clearly states that it is the preferred style (and not all-or-nothing settings), and fallbacking to the variant icon just makes sense. Whatever happens, even not your preferred style is better than missing icon designs on every buttons, etc.