Remove plain .gz .bz2 .xz from the extensions the file-compressor
plug-in registers, and make sure is only ends up in "save_procs", so
at least "exporting" to foo.xcf.gz is not possible any longer. With
some effort, one can still "save" to foo.png.gz tho, but it's much
harder now.
This commit causes a compiler warning because string literals are of
type signed char, and we use a character out of the signed range in the
mimetype for xz. It can be fixed by changing the data type of the mime
string to (unsigned char *), and not using string literals, but that's
a bit too much for this commit.