automatic ViewBox creation

You can make SVG graphics that are automatically zoomed
to take over available space by adding the attributes
viewBox="0 0 width height" and
preserveAspectRatio="xMidYMid meet" to the main SVG
element, where length and height are as specified by
their attributes in said element . Please include a
checkbox to do so in the document settings.

Also, since there's an apparent bug in Inkview that
makes the main clip path not set properly (content
leaks over page edge), the checkbox should also insert
the attributes overflow="hidden" and
clip-path="url(#mainclip)" to the outermost SVG
element, and the following code to the defs section:

<clipPath id="mainclip">
  <path
    id="dummypath1"
    d="M 0,0 L width,0 L width,height L 0,height L 0,0"
  />
</clipPath>

This works and properly hides the out-of-page content
in Inkview.