Add Find7z & Findgzip
Used currently in plasma-frameworks to find gzip (or 7z for Windows) to do svg->svgz conversion on installation.
Planned to use also for libkmahjongg and its svg resources (tiles, background), other places might want to do the same. Ideally there would be also some cmake macro for that, but myself still need to work on that. Having the find modules in ECM already is a start.