Tiled Galleries: use alt value instead of title for alt attribute

I added alt attributes to Tiled Galleries in e75fecaa818a3c84fcc9b64e729fe300bad869c0
but should have used the actual image alt instead of the image title


Reported here:
- http://wordpress.org/support/topic/alt-tag-in-tile-gallery?replies=15#post-5198784
- http://wordpress.org/support/topic/tiled-galleries-alt-attributes-part-2