D20499: Support gzipping of svgz also on windows, using 7z

[Friedrich W. H. Kossebau]

kossebau added reviewers: Plasma, Windows.

REPOSITORY
  R242 Plasma Framework (Library)

REVISION DETAIL
  https://phabricator.kde.org/D20499

To: kossebau, #plasma, #windows
Cc: kde-frameworks-devel, michaelh, ngraham, bruns