Export svg in xhtml, not svgz (bug 10160)

Export svg in xhtml, not svgz (bug 10160)

LyX did not distinguish compressed and uncompressed svg files previously.
Therefore XHTML export of vector graphics did use svgz images directly, which
is not supported by browsers. If svg and svgz are treated as two formats then
all works fine. This is also consistent with the loadable image formats
reported by qt: It reports both svg and svgz.
The gunzip dependency in converters is not new (it is already used internally),
but the gzip dependency is new, so it might not be available on windows.
This is not important at the moment, since we do not yet need to convert svg
to svgz, I only added the converter for completeness.