Subsilver2: apply [code] select mod

[http://shadowm.rewound.net/blog/archives/10-code-selection-with-subsilver2.html]

Commits in master:
 * 9ff21b88049eae041223e2b18fa7b336469889bd
 * c1edeb73ff21387a80b1d09a022a18d24dd1d0a1
AcidTech: apply [code] select mod

[http://shadowm.rewound.net/blog/archives/10-code-selection-with-subsilver2.html]
prosilver: Make [code] blocks expandable/collapsible using Javascript

This commit is the basis for future commits targetting subsilver2 and
acidtech because of the addition of two language constants.
prosilver: Don't reset collapsible code blocks' height when collapsing

This is redundant because height is set to 'auto' when expanding. Also,
offsetHeight is apparently supposed to be a read-only property.
Subsilver2: Set maximum code box height to 200px

Matches prosilver.
AcidTech: Set maximum code box height to 200px

Matches prosilver.
Subsilver2: Make [code] blocks expandable/collapsible using Javascript
AcidTech: Make [code] blocks expandable/collapsible using Javascript
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.
subsilver2: Show codebox expand/collapse buttons only where necessary

Based on "Show codebox expand/collapse buttons only where necessary in
prosilver" (85363ebb6474cff6de84e152a0aa2f034568cf32).
acidtech: Show codebox expand/collapse buttons only where necessary

Based on "Show codebox expand/collapse buttons only where necessary in
prosilver" (85363ebb6474cff6de84e152a0aa2f034568cf32).