Fix item placement problems in dynamic ribbons, and some minor corrections

There was an item placement problem with dynamic ribbons that showed up when there was more than one row. As far as I know, the only case where this could happen was the GP editor. For example: if there were 9 different GPs and a dynamic ribbon with 2 rows and 4 columns, the leftmost GPs showed screenshots of other GPs.

This pull request fixes that problem, and also adds some related minor corrections.