function.dd: Fix accidental heading removal

"Function Parameters" was (accidentally?) removed in commit
26d4aec9cdda3ee07187b6914bb3bd391a3f07c2, resulting in broken links
(as diagnosed by chmgen).
Use relative paths to codemirror.css

Using absolute paths allows the style link tag to work only on dlang.org,
but not on a local filesystem or e.g. a CHM file.
Fix toolbox button tooltips in certain IE versions

The tooltips were not being rendered due to the CSS DXImageTransform
filter necessary for background gradients is old IE versions.
The filter presumably caused an effect similar to overflow:hidden.
This caused tooltips to not appear in e.g. the CHM.
chmgen: Expand absolute path fixing to images
chmgen: Download external JavaScript resources and include them in the CHM

This avoids unexpected firewall alerts when opening the CHM file.
chmgen: Disable hyphenation

CHM readers use an older IE version, with poor JavaScript performance.
This causes some pages to take quite a bit to render.
Merge pull request #340 from CyberShadow/chm-2013-06-13

CHM and related fixes