GtkIconTheme: Repair symbolic fallback with rtl/ltr

GtkIconTheme: Repair symbolic fallback with rtl/ltr

For symbolic icons, we prefer symbolics in inherited themes over
generic icons in the theme itself. So far this was implemented
by looking at icon_name[0] and looking that up in inherited themes
if it is symbolic. But with automatic rtl/ltr handling, the first
icon  name will always have an -rtl or -ltr suffix, and an icon
with that suffix is not going to exist in most cases. To fix this,
look for shorter icon names too, as long as they are still symbolic.

https://bugzilla.gnome.org/show_bug.cgi?id=737000