kiconloader: use librsvg instead of qtsvg for icon rendering

This has a few effects:

• This substantially improves the rendering of various widespread icon themes, e.g. Adwaita icons and application icons for GNOME apps.

This is closer to average than to worst case on Adwaita symbolic icons with Qt SVG: The same icons being rendered with librsvg:

Colourful icons are also prone to SVG rendering errors due to Qt SVG's intentionally limited subset of SVG.

Before (Qt SVG):

After (librsvg):

• This reduces the barrier to entry to contributing to Breeze icons substantially. New contributors to the Breeze icon style have to avoid a lot of basic SVG elements that programs like Inkscape use as part of their default workflow and instead have to learn how to circumvent the shortcomings of the Qt SVG renderer.