ecm_install_icons: Auto-compressing svg to svgz

ecm_install_icons does not compress plain text svg files. But we still want to ship them as compressed *.svgz for obvious reasons. In plasma themes code this is worked around by having ad-hoc CMake function plasma_install_desktoptheme_svgs. For applets, there is no such function, so icons are stored in-tree as compressed: this is inconvenient to work with and to view diffs (which are rarely meaningful for SVG, but sometimes are nice).