Installer/Web: Reorganize CSS & Generate Library Specific CSS

[Foldex]

Now generates libraryroot.custom.css separately instead of just copying the entire webkit.css.
Result should be cleaner and help avoid conflicts, as these files no longer include css they don't actually need.

CSS files had a slight reorganization as well:

• Number prefixes are removed, since they're rather unneeded and juggling them was annoying. Files that we want placed at the top of file listings can simply be prefixed with _ instead.
• dialogs.css was split into several different files, and now all dialog sections are kept under a separate dialogs subfolder.