Add often used attributes for svg and path element

[jwstegemann]

It is now possible to craft SVG images much easier within fritz2, its element DSL now supports the <path>-Tag and the following attributes:

• xmlns
• fill
• viewBox

svg {
    // quite common for each SVG declaration, so directly supported
    xmlns("http://www.w3.org/2000/svg")
    fill("none")
    viewBox("0 0 24 24")
    // too special, so no explicit wrapping function provided
    custom("circle", "opacity-25") {
        attr("cx", "12")
        attr("cy", "12")
        attr("r", "10")
        attr("stroke", "currentColor")
        attr("stroke-width", "4")
    }
    path {
        attr("fill", "currentColor")
        // `d` is explictily supported by `path` tag
        d("M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z")
    }
}