gthemedicon: Fallback to non-symbolic icons

When doing fallback for symbolic icons, we first shorten the name at dashes
while preserving the -symbolic suffix. But after exhausting that, we should also
try stripping the suffix.

See https://bugzilla.gnome.org/show_bug.cgi?id=708163

Signed-off-by: Olivier Brunel <jjk@jjacky.com>

https://bugzilla.gnome.org/show_bug.cgi?id=710254