Fix img title attribute for layouts with thumbnails

For the Exhibit Builder layouts with thumbnails, the value displayed
on mouseover of a thumbnail was the name of the file,
not the name of the item. For most browsers, mouseover will diplay
the value of the title attribute for the img element.
After the fix, both the title and alt attributes for the thumbnail
img contains the title of the associated item.
Add helper function to wrap call to files_for_item

The helper function basically just wraps a call to files_for_item.
Since all themes called files_for_items with the same arguments,
it is easier to wrap the function call in this helper function,
and have the themes call the helper function. In case we need to
add/change an argument to the files_for_item function call,
we just have to change it here, instead of having to do it in each theme.