core: image convertion + SVG support: /!\ new optional dependency: CairoSVG (with installed `[SVG]` extra) - new `convert` method in `tools.image` to save an image in an other format, with support for SVG (when CairoSVG is available) - new `imageConvert` method is available for frontends

You'll find install instructions in documentation (see doc/installation.rst).

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