icontheme: search first in all themes if the requested icon is symbolic

icontheme: search first in all themes if the requested icon is symbolic

Symbolic icons use a "-symbolic" suffix to distinguish themselves from
highcolor variants. Note that the dash character here has a different
meaning than the specificity level defined in the icon-naming-spec [1],
as it identifies a property of the icon itself.

Since they might be provided by a parent theme (e.g. the HighContrast theme
relies on the gnome icon theme for them), when we are looking up one we
should first escape the generic icon inheritance mechanism defined in the
icon-naming-spec [1], and privilege a symbolic icon, if it exists in a
parent theme, before applying the inheritance evaluation.

This fixes symbolic icons not working properly when used in the
HighContrast theme with the GTK_ICON_LOOKUP_GENERIC_FALLBACK flag set.

[1]
http://standards.freedesktop.org/icon-naming-spec/latest/ar01s03.html

https://bugzilla.gnome.org/show_bug.cgi?id=674806