A *temporary* work-around for .xz suffix in kerfuffle/archive.cpp, so that user doesn't prompt to choose "LZMA archive" when open ".xz" file. @reviewers: just revert it if you don't like this work-around.
Add "application/x-lzma-compressed-tar" to libarchive's support mimetypes,since libarchive>=2.6 has optional lzma support,so that ark works with .tar.xz and .tar.lzma with proper libarchive package.

svn path=/trunk/KDE/kdeutils/ark/; revision=955244