Use plain-text title attribute

Since the image caption can contain rich content (in the simplest case, a
page link to the image or another article), using the element's HTML
content will show HTML tags within the title text.

Instead, use the combined text content (with whitespace trimmed) as this
will show the content as plain text.

Change-Id: I5bcc7adcca5a9a2ff953a42f4174245e8701abaf