Use QSvgRenderer for SVG icons (#247)
Use QSvgRenderer for SVG icons (#247)

* Use QSvgRenderer for SVG icons

`QSvgRenderer` is used with a cache for both ordinary and colorized SVG icons. In this way, LXQt's icon handling cannot be broken by intruding icon engines, which register themselves for "svg" (see https://github.com/lxqt/libqtxdg/issues/246). Moreover, it does not depend on a specific code structure of `qtsvg` or any other icon engine.

* Just removed a redundant computation

* Used the same key structure for SVG cache
libqtxdg/libqtxdg,lxde/libqtxdg,lxde/libqtxdg,libqtxdg/libqtxdg