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.

((Revised patch including the following fixes from the same author -- shadowm))

 * Fix attempted assignment to rvalue.
 * Strip “unnecessary compatibility things” from `toggleCodeExpand`.
 * Fix other attempted assignment to rvalue.
 * Check for vertical scrollbar, not horizontal.
   How I missed that, I don’t know.
 * Check whether clientHeight < scrollHeight, not ≠.
   In case scrollHeight can be < clientHeight if a codebox’s code is
   shorter than the codebox’s box.
 * Assign `" &#8226; "` to `df.innerHTML`, not `df.textContent`.
 * Return `false` from `toggleCodeExpand`.
 * Fix `•` generation for what I hope to be the last time.