COMMON: Remove BOOST_FILESYSTEM_VERSION == 2 compatibility

Obsolete in Boost 1.48.0.
COMMON: Vastly simplify FileList

Instead of list of structs containing a path string, a
boost::filesystem::path and a base path string, it is now only
a list of path strings. We never used the rest anyway.
COMMON: Add UString::sless(), case sensitive comparator
COMMON: Remove FileList::getFileNames()

Instead, the FileList::const_iterator should be used.
COMMON: Restructure UString::tolower()/toupper()

- toLower()/toUpper() returns a converted copy
- makeLower()/makeUpper() directly modifies the string
COMMON: Remove FileList::openFile()

We don't use it; we don't need it.
COMMON: Restructure FileList::getSubList()/contains()/findFirst()

There's now getSubList()/contains()/findFirst() and
getSubListGlob()/containsGlob()/findFirstGlob(), both capable
of ignoring case.
AURORA: Use non-glob FileList methods where appropriate
ENGINES: Use non-glob FileList methods where appropriate
JANITORIAL: isEmpty() -> empty()

For consistency's sake.
GRAPHICS: Remove unused variable xMax