QTextCodec initialization is very slow due to it's broken caching in Qt when
ICU is enabled. Make TetCodec initialize fast anyway as it may be used very
often in IMG maps (every LBL subfile has it's own codec!).
Archlinux has started to use the Qt6 build, so to support the legacy East Asian
encodings use the compat module rather than a "clean" but less functional
fallback solution and hope there will be some replacement in Qt6 before
the module gets dropped. Non-UTF XML files will however still not work.
This way we can support East Asian encodings at least on Qt5 in some way
(the Qt codecs are not 100% compatible with the CPx encodings, but the result
should be much better than a fallback to cp1250...).
