Fix layout inside page margin boxes - Since we use flexbox to emulate `vertical-align` behavior in page margin boxes, the content should be wrapped in an element (`span`). Otherwise, when there are a text and an image inside the box, each occupies a single row, resulting a line break between them.
Fix layout inside page margin boxes
- Since we use flexbox to emulate `vertical-align` behavior in page margin boxes, the content should be wrapped in an element (`span`). Otherwise, when there are a text and an image inside the box, each occupies a single row, resulting a line break between them.

vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js