Canvas:Prop layout = centered

Side effect of the code which I hadn't noticed. In practice I feel this one will be truly unavoidable if we do want to fix the Show code button positioning. Either: We allow overlapping (WCAG violation) We introduce systematic vertical padding to prevent overlapping (ActionLogger approach) and lose true centering We use the flex approach in this PR to push the show code button only when the Preview needs the width; we also lose true centering @ghengeveld @MichaelArestad thoughts? I think I tricked myself into thinking that the ideal solution for SyntaxHighlighter (option 3 above) is also ideal for Preview but that's not the case.