Show codebox expand/collapse buttons only where necessary in prosilver

In the prosilver theme, only show a “Expand”/“Collapse” text-button on a
`[code]` box if the box has a scrollbar.

See also <http://forums.wesnoth.org/viewtopic.php?t=38060>.

I implemented translatability of “Expand”/“Collapse” labels via
`CodeboxExpandText` and `CodeboxCollapseText` global variables in
`{overall,simple}_header.html`, as mentioned in the aforelinked thread.